ramips: add support for ELECOM WRC-1167GS2-B

ramips: add support for ELECOM WRC-1167GS2-B

ELECOM WRC-1167GS2-B is a 2.4/5 GHz band 11ac (Wi-Fi 5) router, based
on MT7621A.

Specification:

- SoC		: MediaTek MT7621A
- RAM		: DDR3 128 MiB
- Flash		: SPI-NOR 16 MiB
- WLAN		: 2.4/5 GHz 2T2R (MediaTek MT7615D)
- Ethernet	: 10/100/1000 Mbps x5
  - Switch	: MediaTek MT7530 (SoC)
- LED/keys	: 6x/6x (2x buttons, 1x slide-switch)
- UART		: through-hole on PCB
  - J4: 3.3V, GND, TX, RX from ethernet port side
  - 57600n8
- Power		: 12VDC, 1A

MAC addresses:

LAN	: 04:AB:18:**:**:13 (Factory, 0xFFF4 (hex))
WAN	: 04:AB:18:**:**:14 (Factory, 0xFFFA (hex))
2.4 GHz	: 04:AB:18:**:**:15 (none)
5 GHz	: 04:AB:18:**:**:16 (Factory, 0x4 (hex))

Flash instruction using factory image:

1. Boot WRC-1167GS2-B 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
4. Wait ~120 seconds to complete flashing

Notes:

- there is no way to configure the correct MAC address for secondary phy
  (5GHz) on MT7615D
- Wi-Fi band on primary phy (2.4GHz) cannot be limitted by specifying
  ieee80211-freq-limit
  (fail to register secondary phy due to error)
- mtd-mac-address in the wifi node is required for using
  mtd-mac-address-increment

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
[rebase onto split DTSI patch]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>