ramips: add support for I-O DATA WN-AX2033GR

ramips: add support for I-O DATA WN-AX2033GR

I-O DATA WN-AX2033GR is roughly the same as I-O DATA
WN-AX1167GR2. The difference is Wi-Fi feature.

Specification
=============
- SoC: MediaTek MT7621A
- RAM: DDR3 128 MiB
- Flash Memory: NAND 128 MiB (Spansion S34ML01G200TF100)
- Wi-Fi: MediaTek MT7603E
- Wi-Fi: MediaTek MT7615
- Ethernet: 5x 10 Mbps / 100 Mbps / 1000 Mbps (1x WAN, 4x LAN)
- LED: 2x green LED
- Input: 2x tactile switch, 1x slide switch
- Serial console: 57600bps, PCB through hole J5 (Vcc, TX, RX, NC, GND)
- Power: DC 12V

This device only supports channel 1-13 and 36-140.
Thus, narrower frequency limits compared to other devices are required
for limiting wi-fi frequency correctly.
Without this, non-supported frequencies are activated.

Flash instructions
==================
1. Open the router management page (192.168.0.1).
2. Update router firmware using "initramfs-kernel.bin".
3. After updating, run sysupgrade with "sysupgrade.bin".

Recovery instructions
=====================
WN-AX2033GR contains Zyxel Z-LOADER
1. Setup TFTP server (IP address: 10.10.10.3).
2. Put official firmware into TFTP server directory (distribution site:
   https://www.iodata.jp/lib/software/w/2068.htm)
3. Connect WX-AX2033GR Ethernet port and computer that runs TFTP server.
4. Connect to serial console.
5. Interrupt booting by Esc key.
6. Flash firmware using "ATNR 1,[firmware filename]" command.

Signed-off-by: Yanase Yuki <dev@zpc.sakura.ne.jp>
[adjust for kernel 5.4, add recovery instructions/frequency comment]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>