ramips: add support for Tenbay WR1800K

ramips: add support for Tenbay WR1800K

Hardware
--------
MediaTek MT7621 SoC
256M DDR3
16MB BoHong SPI-NOR
MediaTek MT7905+7975 2x2T2R DBDC bgnax / acax
RGB LED
WPS + RESET Button
UART on compute module (silkscreened / 115200n8)

The router itself is just a board with Power / USB / RJ-45 connectors
and DC/DC converters. The SoC and WiFi components are on a
daughterboard which connect using two M.2 connectors.

The compute module has the model number "T-CB1800K-DM2 V02" printed on
it. The main baord has "T-MB5EU V01" printed on it. This information
might be useful, as it's highly likely either of these two will be
reused in similar designs.

The router itself is sold as Tenbay WR1800K directly from the OEM as
well as "KuWFI AX1800 Smart WiFi 6 Eouter" on Amazon.de for ~50€ in a
slightly different case.

Installation
------------

A Tool for creating a factory image for the Vendor Web Interface can be
found here: https://github.com/blocktrron/wr1800k-factory-creator

Alternatively, the board can be flashed by attaching to the UART
console, interrupting the boot process by keeping "0" pressed while
attaching power.

Serve the OpenWrt initramfs using a TFTP server with address
192.168.1.66. Rename the initramfs to ax1800.bin.

Attach your TFTP server to one of the LAN ports. Execute the following
commands.

 $ setenv ipaddr 192.168.1.67
 $ setenv serverip 192.168.1.66
 $ tftpboot 0x84000000 ax1800.bin
 $ bootm

Wait for the device to boot. Then transfer the OpenWrt sysupgrade image
to the device using SCP and apply sysupgrade.

Signed-off-by: David Bauer <mail@david-bauer.net>