ramips: add support for TP-Link MR6400 v4

You can flash via tftp recovery:
 - serve tftp-recovery image as /tp_recovery.bin on 192.168.0.225/24
 - connect to any ethernet port
 - power on the device while holding the reset button
 - wait at least 8 seconds before releasing reset button

Flashing via OEM web interface does not work.

LTE module does not support DHCP so it must be configured via QMI.

Hardware Specification (v4.0 EU):
 - SoC: MT7628NN
 - Flash: Winbond W25Q64JVS (8MiB)
 - RAM: ESMT M14D5121632A (64MiB)
 - Wireless: SoC platform only (2.4GHz b/g/n, 2x internal antenna)
 - Ethernet: 1NIC (4x100M)
 - WWAN: TP-LINK LTE MODULE (2x external detachable antenna)
 - Power: DC 9V 0.85A

Signed-off-by: Filip Moc <lede@moc6.cz>

target/linux/ramips/dts/mt7628an_tplink_tl-mr6400-v4.dts

@@ -0,0 +1,97 @@
+#include "mt7628an_tplink_8m.dtsi"
+
+/ {
+	compatible = "tplink,tl-mr6400-v4", "mediatek,mt7628an-soc";
+	model = "TP-Link TL-MR6400 v4";
+
+	aliases {
+		led-boot = &led_power;
+		led-failsafe = &led_power;
+		led-running = &led_power;
+		led-upgrade = &led_power;
+	};
+
+	keys {
+		compatible = "gpio-keys";
+
+		reset {
+			label = "reset";
+			gpios = <&gpio 38 GPIO_ACTIVE_LOW>;
+			linux,code = <KEY_RESTART>;
+		};
+
+		rfkill {
+			label = "rfkill";
+			gpios = <&gpio 46 GPIO_ACTIVE_LOW>;
+			linux,code = <KEY_RFKILL>;
+		};
+	};
+
+	leds {
+		compatible = "gpio-leds";
+
+		led_power: power {
+			label = "white:power";
+			gpios = <&gpio 37 GPIO_ACTIVE_LOW>;
+		};
+
+		wan {
+			label = "white:wan";
+			gpios = <&gpio 39 GPIO_ACTIVE_LOW>;
+		};
+
+		wlan {
+			label = "white:wlan";
+			gpios = <&gpio 40 GPIO_ACTIVE_LOW>;
+			linux,default-trigger = "phy0tpt";
+		};
+
+		lan {
+			label = "white:lan";
+			gpios = <&gpio 41 GPIO_ACTIVE_LOW>;
+		};
+
+		signal1 {
+			label = "white:signal1";
+			gpios = <&gpio 42 GPIO_ACTIVE_LOW>;
+		};
+
+		signal2 {
+			label = "white:signal2";
+			gpios = <&gpio 43 GPIO_ACTIVE_LOW>;
+		};
+
+		signal3 {
+			label = "white:signal3";
+			gpios = <&gpio 44 GPIO_ACTIVE_LOW>;
+		};
+	};
+};
+
+&ehci {
+	status = "okay";
+};
+
+&ohci {
+	status = "okay";
+};
+
+&state_default {
+	gpio {
+		groups = "p0led_an", "p1led_an", "p2led_an", "p3led_an", "p4led_an", "refclk", "uart1", "wdt", "wled_an";
+		function = "gpio";
+	};
+};
+
+&esw {
+	mediatek,portmap = <0x2f>;
+	mediatek,portdisable = <0x21>;
+};
+
+&wmac {
+	mtd-mac-address = <&factory 0x1f100>;
+};
+
+&ethernet {
+	mtd-mac-address = <&factory 0x1f100>;
+};