ramips: add support for I-O DATA WN-AC733GR3

ramips: add support for I-O DATA WN-AC733GR3

I-O DATA WN-AC733GR3 is a 2.4/5 GHz band 11ac router, based on
MediaTek MT7620A.

Specification

- SoC		: MediaTek MT7620A
- RAM		: DDR2 64 MiB
- Flash		: SPI-NOR 8 MiB
- WLAN		: 2.4/5 GHz
  - 2.4 GHz : MT7620A (SoC), 2T2R
  - 5 GHz   : MT7610E, 1T1R
- Ethernet	: 10/100/1000 Mbps (RTL8367RB)
- LED/key	: 4x/4x (2x buttons, 1x slide-switch)
- UART		: through-hole on PCB
  - J1: Vcc, RX, GND, TX from LED side
  - 57600n8

Flash instruction using factory image:

1. Boot WN-AC733GR3 normaly
2. Access to "http://192.168.0.1/" and open firmware update page
("ファームウェア")
3. Select the OpenWrt factory image and click update ("更新") button
to perform firmware update
4. Wait ~150 seconds to complete flashing

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>