ramips: add support for H3C TX1800 Plus / TX1801 Plus / TX1806

ramips: add support for H3C TX1800 Plus / TX1801 Plus / TX1806

H3C TX180x series WiFi6 routers are customized by different carrier.
While these three devices look different, they use the same motherboard
inside. Another minor difference comes from the model name definition
in the u-boot environment variable.

Specifications:
 SOC:      MT7621 + MT7915
 ROM:      128 MiB
 RAM:      256 MiB
 LED:      status *2
 Button:   reset *1 + wps/mesh *1
 Ethernet:        lan *3 + wan *1 (10/100/1000Mbps)
 TTL Baudrate:    115200
 TFTP server IP:  192.168.124.99

MAC Address:
 use        address(sample 1)   address(sample 2)    source
 label      88:xx:xx:98:xx:12   88:xx:xx:a2:xx:a5   u-boot-env@ethaddr
 lan        88:xx:xx:98:xx:13   88:xx:xx:a2:xx:a6   $label +1
 wan        88:xx:xx:98:xx:12   88:xx:xx:a2:xx:a5   $label
 WiFi4_2G   8a:xx:xx:58:xx:14   8a:xx:xx:52:xx:a7   (Compatibility mode)
 WiFi5_5G   8a:xx:xx:b8:xx:14   8a:xx:xx:b2:xx:a7   (Compatibility mode)
 WiFi6_2G   8a:xx:xx:18:xx:14   8a:xx:xx:12:xx:a7
 WiFi6_5G   8a:xx:xx:78:xx:14   8a:xx:xx:72:xx:a7

Compatibility mode is used to guarantee the connection of old devices
that only support WiFi4 or WiFi5.

TFTP + TTL Installation:
Although a TTL connection is required for installation, we do not need
to tear down it. We can find the TTL port from the cooling hole at the
bottom. It is located below LAN3 and the pins are defined as follows:
|LAN1|LAN2|LAN3|----|WAN|
--------------------
    |GND|TX|RX|VCC|

1. Set tftp server IP to 192.168.124.99 and put initramfs firmware in
   server's root directory, rename it to a simple name "initramfs.bin".
2. Plug in the power supply and wait for power on, connect the TTL cable
   and open a TTL session, enter "reboot", then enter "Y" to confirm.
   Finally push "0" to interruput boot while booting.
3. Execute command to install a initramfs system:
   # tftp 0x80010000 192.168.124.99:initramfs.bin
   # bootm 0x80010000
4. Backup nand flash by OpenWrt LuCI or dd instruction. We need those
   partitions if we want to back to stock firmwre due to official
   website does not provide download link.
   # dd if=/dev/mtd1 of=/tmp/u-boot-env.bin
   # dd if=/dev/mtd4 of=/tmp/firmware.bin
5. Edit u-boot env to ensure use default bootargs and first image slot:
   # fw_setenv bootargs
   # fw_setenv bootflag 0
6. Upgrade sysupgrade firmware.
7. About restore stock firmware: flash the "firmware" and "u-boot-env"
   partitions that we backed up in step 4.
   # mtd write /tmp/u-boot-env.bin u-boot-env
   # mtd write /tmp/firmware.bin firmware

Additional Info:
The H3C stock firmware has a 160-byte firmware header that appears to
use a non-standard CRC32 verification algorithm. For this part of the
data, the u-boot does not check it so we can just directly replace it
with a placeholder.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>