ramips: add support for ipTIME AX2004M

ipTIME AX2004M is an 802.11ax (Wi-Fi 6) router, based on MediaTek
MT7621A.

Specification:
* SoC: MT7621A
* RAM: 256 MiB
* Flash: NAND 128 MiB
* Wi-Fi:
  * MT7915D: 2.4/5 GHz (DBDC)
* Ethernet: 5x 1GbE
  * Switch: SoC built-in
* USB: 1x 3.0
* UART: J4 (115200 baud)
  * Pinout: [3V3] (TXD) (RXD) (GND)

MAC address:

| interface |        MAC        |     source     | comment
|-----------|-------------------|----------------|---------
|       LAN | 58:XX:XX:00:XX:9B |                | [1]
|       WAN | 58:XX:XX:00:XX:99 |                |
|   WLAN 2G | 58:XX:XX:00:XX:98 | factory 0x4    |
|   WLAN 5G | 5A:XX:XX:40:XX:98 |                |
|           |                   |                |
|           | 58:XX:XX:00:XX:98 | config ethaddr |

[1] Used in this patch as WLAN 5G MAC address with the local bit set

Load address:
* stock
  * 0x80010000: FIT image
  * 0x81001000: kernel image -> entry
* OpenWrt
  * 0x80010000: FIT image
  * 0x82000000: uncompressed kernel+relocate image
  * 0x80001000: relocated kernel image -> entry

Installation via **recovery** mode:
1.  Press reset button, power up the device, wait >10s for CPU LED
    to stop blinking.
2.  Upload recovery image through the recovery web page at 192.168.0.1.

Revert to stock firmware:
1.  Install stock image via recovery mode.

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

target/linux/ramips/dts/mt7621_iptime_ax2004m.dts

@@ -0,0 +1,165 @@
+// SPDX-License-Identifier: GPL-2.0-or-later OR MIT
+
+#include "mt7621.dtsi"
+
+#include <dt-bindings/gpio/gpio.h>
+#include <dt-bindings/input/input.h>
+
+/ {
+	compatible = "iptime,ax2004m", "mediatek,mt7621-soc";
+	model = "ipTIME AX2004M";
+
+	aliases {
+		led-boot = &led_cpu;
+		led-failsafe = &led_cpu;
+		led-running = &led_cpu;
+		led-upgrade = &led_cpu;
+	};
+
+	chosen {
+		bootargs = "console=ttyS0,115200";
+	};
+
+	leds {
+		compatible = "gpio-leds";
+
+		led_cpu: cpu {
+			label = "amber:cpu";
+			gpios = <&gpio 18 GPIO_ACTIVE_LOW>;
+		};
+
+		wlan2g {
+			label = "amber:wlan2g";
+			gpios = <&gpio 17 GPIO_ACTIVE_LOW>;
+			linux,default-trigger = "phy0radio";
+		};
+
+		wlan5g {
+			label = "amber:wlan5g";
+			gpios = <&gpio 14 GPIO_ACTIVE_LOW>;
+			linux,default-trigger = "phy1radio";
+		};
+	};
+
+	keys {
+		compatible = "gpio-keys";
+
+		reset {
+			label = "reset";
+			gpios = <&gpio 4 GPIO_ACTIVE_LOW>;
+			linux,code = <KEY_RESTART>;
+		};
+	};
+};
+
+&nand {
+	status = "okay";
+
+	partitions {
+		compatible = "fixed-partitions";
+		#address-cells = <1>;
+		#size-cells = <1>;
+
+		partition@0 {
+			label = "u-boot";
+			reg = <0x0 0x80000>;
+			read-only;
+		};
+
+		partition@80000 {
+			label = "config";
+			reg = <0x80000 0x80000>;
+			read-only;
+		};
+
+		factory: partition@100000 {
+			label = "factory";
+			reg = <0x100000 0x80000>;
+			read-only;
+
+			compatible = "nvmem-cells";
+			#address-cells = <1>;
+			#size-cells = <1>;
+
+			macaddr_factory_4: macaddr@4 {
+				reg = <0x4 0x6>;
+			};
+		};
+
+		partition@180000 {
+			label = "firmware";
+			reg = <0x180000 0x7680000>;
+
+			compatible = "fixed-partitions";
+			#address-cells = <1>;
+			#size-cells = <1>;
+
+			partition@0 {
+				label = "kernel";
+				reg = <0x0 0x400000>;
+			};
+
+			partition@400000 {
+				label = "ubi";
+				reg = <0x400000 0x7280000>;
+			};
+		};
+	};
+};
+
+&state_default {
+	gpio {
+		groups = "i2c", "jtag", "wdt";
+		function = "gpio";
+	};
+};
+
+&gmac0 {
+	nvmem-cells = <&macaddr_factory_4>;
+	nvmem-cell-names = "mac-address";
+	mac-address-increment = <3>;
+};
+
+&switch0 {
+	ports {
+		port@0 {
+			status = "okay";
+			label = "wan";
+			nvmem-cells = <&macaddr_factory_4>;
+			nvmem-cell-names = "mac-address";
+			mac-address-increment = <1>;
+		};
+
+		port@1 {
+			status = "okay";
+			label = "lan4";
+		};
+
+		port@2 {
+			status = "okay";
+			label = "lan3";
+		};
+
+		port@3 {
+			status = "okay";
+			label = "lan2";
+		};
+
+		port@4 {
+			status = "okay";
+			label = "lan1";
+		};
+	};
+};
+
+&pcie {
+	status = "okay";
+};
+
+&pcie1 {
+	wifi@0,0 {
+		compatible = "mediatek,mt76";
+		reg = <0x0000 0 0 0 0>;
+		mediatek,mtd-eeprom = <&factory 0x0>;
+	};
+};