ramips: add support for Buffalo WSR-2533DHPL2

ramips: add support for Buffalo WSR-2533DHPL2

Buffalo WSR-2533DHPL2 is a 2.4/5 GHz band 11ac (Wi-Fi 5) router, based
on MediaTek MT7621A.

Specification:

- SoC           : MediaTek MT7621AT
- RAM           : DDR3 128 MiB (Winbond W631GG6MB12J)
- Flash         : RAW-NAND 128 MiB (Winbond W29N01HVSINF)
- WLAN          : 2.4/5 GHz (2x MediaTek MT7615N)
- Ethernet      : 10/100/1000 Mbps x4
  - Switch      : MediaTek MT7530 (SoC)
- LED/keys      : 8x/6x (2x buttons, 1x slide-switch)
- UART          : through-hole on PCB (J4)
  - arrangement : 3.3V, GND, TX, RX from triangle-mark
  - settings    : 57600n8
- Power         : 12VDC 1.5A

Flash instruction using factory.bin image:

1. boot WSR-2533DHPL2 normally with "Router" mode
2. access to the WebI ("http://192.168.11.1/") on the device and open
   firmware update page
   ("管理" -> "ファームウェア更新")
3. select the OpenWrt factory.bin image and click update ("更新実行")
   button
   Attention: do not use "factory-uboot.bin" image
4. Wait ~120 seconds to complete flashing

Flash instruction using initramfs image:

1. prepare the TFTP server with the initramfs image renamed to
   "linux.trx-recovery" and IP address "192.168.11.2"
2. press the "AOSS" button while powering on the WSR-2533DHPL2
3. after 10 seconds, release the "AOSS" button, WSR-2533DHPL2 downloads
   the initramfs image and boot with it automatically
4. on the initramfs image, download the factory-uboot.bin image to the
   device and perform sysupgrade with it and "-F" option
5. wait ~120 seconds to complete flashing

Notes:

- There are 2x factory*.bin images for different purposes.

  - factory.bin      : for flashing on OEM WebUI
  - factory-uboot.bin: for flashing on OEM bootloader or initramfs image

  factory-uboot.bin is useful for recoverying the device, or refreshing
  when the kernel partition is expanded in the future. sysupgrade on
  this device accepts factory-uboot.bin with option "-F", but on that
  situation, user configurations won't be kept, so it's not for normal
  use.

MAC addresses:

LAN    : 18:EC:E7:xx:xx:E0 (board_data, "mac" (text))
WAN    : 18:EC:E7:xx:xx:E0 (board_data, "mac" (text))
2.4 GHz: 18:EC:E7:xx:xx:E1 (Factory, 0x4    (hex))
5 GHz  : 18:EC:E7:xx:xx:E4 (Factory, 0x8004 (hex))

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