ramips: add support for I-O DATA WN-GX300GR

ramips: add support for I-O DATA WN-GX300GR

I-O DATA WN-GX300GR is a 2.4 GHz band 11n router, based on MediaTek
MT7621S.

Specification:

- MT7621S (1-Core, 2-Threads)
- 64 MB of RAM
- 8 MB of Flash (SPI)
- 2T2R 2.4 GHz
- 5x 10/100/1000 Mbps Ethernet
- 2x LEDs, 4x keys (2x buttons, 1x slide switch)
- UART header on PCB
  - Vcc, GND, TX, RX from ethernet port side
  - baudrate: 115200 bps (U-Boot, OpenWrt)

Flash instruction using initramfs image:

1. Connect serial cable to UART header
2. Rename OpenWrt initramfs image for WN-GX300GR to "uImageWN-GX300GR"
and place it in the TFTP directory
3. Set the IP address of the computer to 192.168.99.8, connect to the
LAN port of WN-GX300GR, and start the TFTP server on the computer
4. Connect power cable to WN-GX300GR and turn on the router
5. Press "1" key on the serial console to interrupt boot process on
U-Boot, press Enter key 3 times and start firmware download via TFTP
6. WN-GX300GR downloads initramfs image and boot with it
7. On the initramfs image, execute "mtd erase firmware" to erase stock
firmware and execute sysupgrade with sysupgrade image for WN-GX300GR
8. Wait ~150 seconds to complete flasing

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