ramips: add support for ELECOM WRC-2533GHBK2-T

ramips: add support for ELECOM WRC-2533GHBK2-T

ELECOM WRC-2533GHBK2-T 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 (Macronix MX25L12835FM2I-10G)
- WLAN     : 2.4/5 GHz 4T4R (2x MediaTek MT7615)
- Ethernet : 10/100/1000 Mbps x5
  - Switch : MediaTek MT7530 (SoC)
- LED/keys : 4x/3x (2x buttons, 1x slide-switch)
- UART     : through-hole on PCB
  - J4: 3.3V, RX, GND, TX from SoC side
  - 57600n8
- Power    : 12 VDC, 1.5 A

Flash instruction using factory image:

1. Boot WRC-2533GHBK2-T 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 ~150 seconds to complete flashing

MAC addresses:

LAN    : BC:5C:4C:xx:xx:FD (Config,  "ethaddr" (text))
WAN    : BC:5C:4C:xx:xx:FC (Config,  "wanaddr" (text), Label)
2.4 GHz: BC:5C:4C:xx:xx:FE (Factory, 0x4       (hex))
5 GHz  : BC:5C:4C:xx:xx:FF (Factory, 0x8004    (hex))

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