ramips: add support for netis WF2770

ramips: add support for netis WF2770

netis WF2770 is a 2.4/5GHz band AC750 router, based on MediaTek MT7620A.

Specifications:
- SoC: MT7620A
- RAM: DDR2 64MB
- Flash: SPI NOR 16MB
- WiFi:
  - 2.4GHz: SoC internal
  - 5GHz: MT7610EN
- Ethernet: 5x 10/100/1000Mbps
  - Switch: MT7530BU
- UART:
  - J2: 3.3V, RX, TX, GND (3.3V is the square pad) / 57600 8N1

MAC addresses in factory partition:
0x0004: LAN, WiFi 2.4GHz (label_mac-6)
0x0028: not used (label_mac-1)
0x002e: WAN (label_mac)
0x8004: WiFi 5GHz (label_mac+2)

Installation via web interface:
1.  Flash **initramfs** image through the stock web interface.
2.  Boot into OpenWrt and perform sysupgrade with sysupgrade image.

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

Reviewed-by: Pawel Dembicki <paweldembicki@gmail.com>
Signed-off-by: Sungbo Eo <mans0n@gorani.run>