ramips: add support for ELECOM WRC-1750GS/GSV

ramips: add support for ELECOM WRC-1750GS/GSV

ELECOM WRC-1750GS is a 2.4/5 GHz band 11ac (Wi-Fi 5) router, based on
MT7621A.

WRC-1750GSV has the same hardware with WRC-1750GS.

Specification:

- SoC		: MediaTek MT7621A
- RAM		: DDR3 128 MiB
- Flash		: SPI-NOR 16 MiB
- WLAN		: 2.4/5 GHz 3T3R (2x MediaTek MT7615)
- Ethernet	: 10/100/1000 Mbps x5
  - Switch	: MediaTek MT7530 (SoC)
- LED/keys	: 4x/6x (2x buttons, 1x slide-switch)
- UART		: through-hole on PCB
  - J4: 3.3V, GND, TX, RX from ethernet port side
  - 57600n8
- Power		: 12VDC, 1.5A

Flash instruction using factory image:

1. Boot WRC-1750GS (or WRC-1750GSV) normally
2. Access to "http://192.168.2.1/" and open firmware update page
   ("ファームウェア更新")
3. Select the OpenWrt factory image and click apply ("適用") button
   for WRC-1750GS : wrc-1750gs-squashfs-factory.bin
   for WRC-1750GSV: wrc-1750gsv-squashfs-factory.bin
4. Wait ~120 seconds to complete flashing

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