MIPS: ath79: add initial support for Onion Omega

The following features are supported:

* UART;
* SPI-flash;
* USB host;
* GPIO key and LED.

Please see https://onion.io/omega for details.

Signed-off-by: Antony Pavlov <[e-mail]>
Cc: Gabor Juhos <[e-mail]>
Cc: Alban Bedel <[e-mail]>
Cc: L. D. Pinney <[e-mail]>
Cc: Boken Lin <[e-mail]>
Cc: Jacky Huang <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Patchwork: https://patchwork.linux-mips.org/patch/12884/
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/boot/dts/qca/Makefile

···
 # All DTBs
 dtb-$(CONFIG_ATH79)			+= ar9132_tl_wr1043nd_v1.dtb
 dtb-$(CONFIG_ATH79)			+= ar9331_dragino_ms14.dtb
+dtb-$(CONFIG_ATH79)			+= ar9331_omega.dtb
 dtb-$(CONFIG_ATH79)			+= ar9331_tl_mr3020.dtb
 
 # Force kbuild to make empty built-in.o if necessary

arch/mips/boot/dts/qca/ar9331_omega.dts

···
+/dts-v1/;
+
+#include <dt-bindings/gpio/gpio.h>
+#include <dt-bindings/input/input.h>
+
+#include "ar9331.dtsi"
+
+/ {
+	model = "Onion Omega";
+	compatible = "onion,omega";
+
+	aliases {
+		serial0 = &uart;
+	};
+
+	memory@0 {
+		device_type = "memory";
+		reg = <0x0 0x4000000>;
+	};
+
+	leds {
+		compatible = "gpio-leds";
+
+		system {
+			label = "onion:amber:system";
+			gpios = <&gpio 27 GPIO_ACTIVE_LOW>;
+			default-state = "off";
+		};
+	};
+
+	gpio-keys-polled {
+		compatible = "gpio-keys-polled";
+		#address-cells = <1>;
+		#size-cells = <0>;
+		poll-interval = <100>;
+
+		button@0 {
+			label = "reset";
+			linux,code = <KEY_RESTART>;
+			gpios = <&gpio 11 GPIO_ACTIVE_HIGH>;
+		};
+	};
+};
+
+&ref {
+	clock-frequency = <25000000>;
+};
+
+&uart {
+	status = "okay";
+};
+
+&gpio {
+	status = "okay";
+};
+
+&usb {
+	dr_mode = "host";
+	status = "okay";
+};
+
+&usb_phy {
+	status = "okay";
+};
+
+&spi {
+	num-chipselects = <1>;
+	status = "okay";
+
+	/* Winbond 25Q128FVSG SPI flash */
+	spiflash: w25q128@0 {
+		#address-cells = <1>;
+		#size-cells = <1>;
+		compatible = "winbond,w25q128", "jedec,spi-nor";
+		spi-max-frequency = <104000000>;
+		reg = <0>;
+	};
+};