ramips: add support for I-O DATA WN-AX1167GR2

ramips: add support for I-O DATA WN-AX1167GR2

I-O DATA WN-AX1167GR2 is a 2.4/5 GHz band 11ac router, based on MediaTek
MT7621A.

Specification:

- SoC		: MediaTek MT7621A
- RAM		: DDR3 128 MiB
- Flash		: NAND 128 MiB
- WLAN		: MediaTek MT7615D (2.4/5 GHz, 2T2R)
- Ethernet	: 5x 10/100/1000 Mbps
  - Switch	: MediaTek MT7621A (MT7530)
- LEDs/Input	: 2x/3x (2x buttons, 1x slide-switch)
- UART		: through-hole on PCB
  - J5: Vcc, TX, RX, NC, GND
  - 57600 bps

Flash instruction using initramfs image:

1. Boot WN-AX1167GR2 normally
2. Access to "http://192.168.0.1/" and open firmware update page
("ファームウェア")
3. Select the OpenWrt initramfs image and click update ("更新")
button to perform firmware update
4. On the initramfs image, perform sysupgrade with squashfs-sysupgrade
image
5. Wait ~120 seconds to complete flashing

Notes:

- configuration in DeviceTree of DBDC (Dual-Band-Dual-Concurrent) mode
for MT7615D chip is not supported in mt76 driver
- last 0x80000 (512 KiB) in NAND flash is not used on stock firmware
- stock firmware requires "customized uImage header" by MSTC
(MitraStar Technology Corp.), but U-Boot doesn't
  - uImage magic (0x0 - 0x3) : 0x434F4D42 (COMB)
  - header crc32 (0x4 - 0x7) : with data length and data crc32
  - image name (0x20 - 0x37) : model ID and firmware versions
  - data length (0x38 - 0x3b): kernel + rootfs
  - data crc32 (0x3c - 0x3f) : kernel + rootfs

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Reviewed-by: Sungbo Eo <mans0n@gorani.run>