ramips: add support for ELECOM WMC-C2533GST

ramips: add support for ELECOM WMC-C2533GST

ELECOM WMC-C2533GST is a 2.4/5 GHz band 11ac (Wi-Fi 5) mesh
router, based on MT7621A.

It shares the same basic hardware and 32 MiB flash layout as
WRC-2533GST2, but the factory image uses the WMC-2HC hardware name.

Specification:

- SoC           : MediaTek MT7621A
- RAM           : DDR3 256 MiB
- Flash         : SPI-NOR 32 MiB (Macronix MX25L25635E)
- WLAN          : 2.4/5 GHz 4T4R (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 WMC-C2533GST normally with "Router" mode
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     : 04:AB:18:xx:xx:BF (Factory, 0xFFF4 (hex))
WAN     : 04:AB:18:xx:xx:C0 (Factory, 0xFFFA (hex))
2.4GHz  : 04:AB:18:xx:xx:C1 (Factory, 0x4    (hex))
5GHz    : 04:AB:18:xx:xx:C2 (Factory, 0x8004 (hex))

Signed-off-by: Taiga Ogawa <zectaiga@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23568
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>