ramips: add support for Buffalo WSR-2533DHPL

ramips: add support for Buffalo WSR-2533DHPL

Buffalo WSR-2533DHPL is a 2.4/5 GHz band 11ac router, based on MediaTek
MT7621A.

Specification:

- SoC		: MediaTek MT7621A
- RAM		: DDR3 128 MiB
- Flash		: SPI-NOR 16 MiB
- WLAN		: 2.4/5 GHz 4T4R (2x MediaTek MT7615N)
- Ethernet	: 10/100/1000 Mbps
  - Switch	: MediaTek MT7530 (SoC)
- LED/keys	: 8x/6x (3x buttons, 2x slide-switches)
- UART		: through-hole on PCB
  - J4: 3.3V, GND, TX, RX from triangle-mark
  - 57600n8
- Power		: 12VDC 1.5A

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-2533DHPL
3. after 10 seconds, release the "AOSS" button, WSR-2533DHPL downloads
   the initramfs image and boot with it automatically
4. on the initramfs image, download the sysupgrade image to the device
   and perform sysupgrade with it
5. wait ~120 seconds to complete flashing

Switch position overview:

- slide-switch1 (2x positions)
  - "AUTO"
  - "MANUAL" (not connected to gpio)

- slide-switch2 (3x positions)
  - "ROUTER"
  - "AP" (not connected to gpio)
  - "WB"

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
[add note on switches, fix group->groups for state_default]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>