ramips: add support for WeVO AIR DUO

WeVO AIR DUO is a 1-bay NAS & 802.11ac (Wi-Fi 5) router, based on
MediaTek MT7620A.

Specifications:
* SoC: MT7620A
* RAM: 64 MiB
* Flash: SPI NOR 16 MiB
* USB & SATA bridge controller: JMicron JMS567
  * SATA 6Gb/s: 2.5" drive slot
  * USB 3.0: Micro-B
  * USB 2.0: connected to SoC
* Wi-Fi:
  * 2.4 GHz: SoC built-in
  * 5 GHz: MT7612EN
* Ethernet: 5x 1GbE
  * Switch: MT7530WU
* UART: 4-pin 1.27 mm pitch through-hole (57600 baud)
  * Pinout: (3V3)|(RXD) (TXD) (GND)

Notes:
* The drive is accessible through the external USB port only when the
  router is turned off.

Installation via web interface:
1.  Flash **initramfs** image through the stock web interface.
    The image filename should have ".upload" extension.
2.  Boot into OpenWrt and perform sysupgrade with sysupgrade image.

Revert to stock firmware:
1.  Perform sysupgrade with stock image.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>

target/linux/ramips/dts/mt7620a_wevo_air-duo.dts

@@ -0,0 +1,202 @@
+// SPDX-License-Identifier: GPL-2.0-or-later OR MIT
+
+#include "mt7620a.dtsi"
+
+#include <dt-bindings/gpio/gpio.h>
+#include <dt-bindings/input/input.h>
+#include <dt-bindings/leds/common.h>
+
+/ {
+	compatible = "wevo,air-duo", "ralink,mt7620a-soc";
+	model = "WeVO AIR DUO";
+
+	aliases {
+		led-boot = &led_status;
+		led-failsafe = &led_status;
+		led-running = &led_status;
+		led-upgrade = &led_status;
+		label-mac-device = &ethernet;
+	};
+
+	leds {
+		compatible = "gpio-leds";
+
+		led_status: led-0 {
+			label = "blue:status";
+			color = <LED_COLOR_ID_BLUE>;
+			function = LED_FUNCTION_STATUS;
+			gpios = <&gpio0 11 GPIO_ACTIVE_LOW>;
+		};
+
+		led-1 {
+			color = <LED_COLOR_ID_BLUE>;
+			function = LED_FUNCTION_WPS;
+			gpios = <&gpio1 15 GPIO_ACTIVE_LOW>;
+		};
+	};
+
+	keys {
+		compatible = "gpio-keys";
+
+		wps {
+			label = "wps";
+			gpios = <&gpio0 1 GPIO_ACTIVE_LOW>;
+			linux,code = <KEY_WPS_BUTTON>;
+		};
+
+		reset {
+			label = "reset";
+			gpios = <&gpio0 2 GPIO_ACTIVE_LOW>;
+			linux,code = <KEY_RESTART>;
+		};
+	};
+};
+
+&gpio1 {
+	status = "okay";
+};
+
+&spi0 {
+	status = "okay";
+
+	flash@0 {
+		compatible = "jedec,spi-nor";
+		reg = <0>;
+		spi-max-frequency = <50000000>;
+
+		partitions {
+			compatible = "fixed-partitions";
+			#address-cells = <1>;
+			#size-cells = <1>;
+
+			partition@0 {
+				label = "u-boot";
+				reg = <0x0 0x30000>;
+				read-only;
+			};
+
+			partition@30000 {
+				label = "config";
+				reg = <0x30000 0x10000>;
+				read-only;
+			};
+
+			factory: partition@40000 {
+				label = "factory";
+				reg = <0x40000 0x10000>;
+				read-only;
+
+				compatible = "nvmem-cells";
+				#address-cells = <1>;
+				#size-cells = <1>;
+
+				macaddr_factory_4: macaddr@4 {
+					reg = <0x4 0x6>;
+				};
+			};
+
+			partition@50000 {
+				compatible = "denx,uimage";
+				label = "firmware";
+				reg = <0x50000 0xeb0000>;
+			};
+
+			partition@f00000 {
+				label = "db";
+				reg = <0xf00000 0x100000>;
+				read-only;
+			};
+		};
+	};
+};
+
+&state_default {
+	gpio {
+		groups = "i2c", "uartf", "spi refclk";
+		function = "gpio";
+	};
+};
+
+&ethernet {
+	pinctrl-names = "default";
+	pinctrl-0 = <&rgmii1_pins &mdio_pins>;
+	nvmem-cells = <&macaddr_factory_4>;
+	nvmem-cell-names = "mac-address";
+
+	port@5 {
+		status = "okay";
+		mediatek,fixed-link = <1000 1 1 1>;
+		phy-mode = "rgmii";
+	};
+
+	mdio-bus {
+		status = "okay";
+
+		ethernet-phy@0 {
+			reg = <0>;
+			phy-mode = "rgmii";
+		};
+
+		ethernet-phy@1 {
+			reg = <1>;
+			phy-mode = "rgmii";
+		};
+
+		ethernet-phy@2 {
+			reg = <2>;
+			phy-mode = "rgmii";
+		};
+
+		ethernet-phy@3 {
+			reg = <3>;
+			phy-mode = "rgmii";
+		};
+
+		ethernet-phy@4 {
+			reg = <4>;
+			phy-mode = "rgmii";
+		};
+
+		ethernet-phy@1f {
+			reg = <0x1f>;
+			phy-mode = "rgmii";
+		};
+	};
+};
+
+&gsw {
+	mediatek,ephy-base = /bits/ 8 <12>;
+};
+
+&ehci {
+	status = "okay";
+};
+
+&ohci {
+	status = "okay";
+};
+
+&pcie {
+	status = "okay";
+};
+
+&pcie0 {
+	wifi@0,0 {
+		compatible = "mediatek,mt76";
+		reg = <0x0000 0 0 0 0>;
+		mediatek,mtd-eeprom = <&factory 0x8000>;
+		ieee80211-freq-limit = <5000000 6000000>;
+
+		led {
+			led-sources = <2>;
+			led-active-low;
+		};
+	};
+};
+
+&wmac {
+	pinctrl-names = "default";
+	pinctrl-0 = <&wled_pins>;
+
+	ralink,mtd-eeprom = <&factory 0x0>;
+};