ramips: Added Onion Omega2 and Omega2+

This patch adds support for the Onion Omega2 and Omega2+ (https://onion.io)

Specifications:
- SoC: MediaTek MT7688AN (580MHz, ramips)
- Omega2
  - RAM: 64MB DDR
  - Storage: 16MB NOR SPI flash onboard
- Omega2+
  - RAM: 128MB DDR
  - Storage: 32MB NOR SPI flash onboard + microSD slot
- Wireless: Built into MT7688AN (mt76) with onboard 1x chip antenna and u.FL connecter
- Ethernet: 1x100M pins on Omega2 & Omega2+, can use Ethernet Expansion and an Omega Dock to get a physical Ethernet port
- Strongly recommend using the Omega2 & Omega2+ with a Dock (Expansion Dock, Power Dock, Arduino Dock 2, Mini Dock)
  - All Docks Provide:
    - Micro-USB port to provide power to the Omega
      - On the Expansion and Mini Docks, can also access the terminal (UART0) via serial
    - USB 2.0 socket connected to Omega
  - Just the Expansion Dock, Power Dock, and Arduino Dock 2 provide:
    - Omega GPIO breakout
    - Allows for connection of Omega Expansions:
      - Ethernet Expansion
      - Relay Expansion
      - PWM Expansion
      - OLED Expansion
      - Ethernet Expansion
      - Proto Expansion
      - Cellular Expansion

Signed-off-by: Lazar Demin <lazar@onion.io>

target/linux/ramips/dts/OMEGA2.dts

@@ -0,0 +1,20 @@
+/dts-v1/;
+
+#include "OMEGA2.dtsi"
+
+/ {
+	model = "Onion Omega2";
+
+	memory@0 {
+		device_type = "memory";
+		reg = <0x0 0x4000000>;
+	};
+};
+
+&firmware {
+	reg = <0x50000 0xfb0000>;
+};
+
+&system_led {
+	label = "omega2:amber:system";
+};