ramips: add support for Cudy M1800

ramips: add support for Cudy M1800

Specifications

  SoC:       MT7621
  CPU:       880 MHz
  Flash:     32 MiB
  RAM:       256 MiB
  WLAN:      MT7915 WiFi 6 (2.4/5 GHz)
  Ethernet:  2x Gbit ports

                  MAC
LAN        b4:4b:d6:2e:c7:b0 (label)
WAN        b4:4b:d6:2e:c7:b1
WiFi 2.4   00:0c:43:26:46:08
WiFi 5     00:0c:43:26:59:97

Installation

There are two known options:
1) The Luci-based UI.
2) Press and hold the reset button during power up.
   The router will request 'recovery.bin' from a TFTP server at
   192.168.1.88.

Both options require a signed firmware binary.
The openwrt image supplied by cudy is signed and can be used to
install unsigned images.

Signed-off-by: Leon M. Busch-George <leon@georgemail.eu>