ramips: add support for Z-ROUTER ZR-2660

ramips: add support for Z-ROUTER ZR-2660

This commit adds support for Z-ROUTER ZR-2660 (also known as Routerich
AX1800) wireless WiFi 6 router.

Specification
-------------
- SoC       : MediaTek MT7621AT, MIPS, 880 MHz
- RAM       : 256 MiB
- Flash     : NAND 128 MiB (AMD/Spansion S34ML01G2)
- WLAN      :
  - 2.4 GHz : MediaTek MT7905D/MT7975 (14c3:7916), b/g/n/ax, MIMO 2x2
  - 5 GHz   : MediaTek MT7915E (14c3:7915), a/n/ac/ax, MIMO 2x2
- Ethernet  : 10/100/1000 Mbps x4 (1x WAN, 3x LAN)
- USB       : 1x 2.0
- UART      : 3.3V, 115200n8, pins are silkscreened on the pcb
- Buttons   : 1x Reset
- LEDs      : 1x WiFi 2.4 GHz (green)
              1x WiFi 5 GHz (green)
              1x LAN (green)
              1x WAN (green)
              1x WAN no-internet (red)
- Power     : 12 VDC, 1 A

Installation
------------
1. Run tftp server on your PC (IP: 192.168.2.2) and put OpenWrt initramfs
   image (initramfs.bin) to the tftp root dir
2. Open the following link in the browser to enable telnet:
	http://192.168.2.1/cgi-bin/telnet_ssh
3. Connect to the router (default IP: 192.168.2.1) using telnet shell
   (credentials - user:admin)
4. Run the following commands in the telnet shell (this will install
   OpenWrt initramfs image on nand flash):
	cd /tmp
	tftp -g -r initramfs.bin 192.168.2.2
	mtd write initramfs.bin firmware
	mtd erase firmware_backup
	reboot
5. Copy OpenWrt sysupgrade image (sysupgrade.bin) to the /tmp dir of the
   router
6. Connect to the router (IP: 192.168.1.1) using ssh shell and run
   sysupgrade command:
	sysupgrade -n /tmp/sysupgrade.bin

Return to stock
---------------
1. Copy stock firmware (stock.bin) to the /tmp dir of the router using scp
2. Run following command in the router shell:
	cd /tmp
	mtd write stock.bin firmware
	reboot

Recovery
--------
Connect uart (pins are silkscreened on the pcb), interrupt boot process by
pressing any key, use u-boot menu to flash stock firmware image or OpenWrt
initramfs image.

MAC addresses
-------------
+---------+-------------------+-----------+
|         | MAC               | Algorithm |
+---------+-------------------+-----------+
| LAN     | 24:0f:5e:xx:xx:4c | label     |
| WAN     | 24:0f:5e:xx:xx:4d | label+1   |
| WLAN 2g | 24:0f:5e:xx:xx:4e | label+2   |
| WLAN 5g | 24:0f:5e:xx:xx:4f | label+3   |
+---------+-------------------+-----------+
The WLAN 2.4 MAC was found in 'factory', 0x4
The LAN MAC was found in 'factory', 0xfff4
The WAN MAC was found in 'factory', 0xfffa

Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>