ramips: add support for Cudy X6

ramips: add support for Cudy X6

Specifications:

SoC: MediaTek MT7621
RAM: 256 MB
Flash: 32 MB
WiFi: MediaTek MT7915E
Switch: 1 WAN, 4 LAN (Gigabit)
Ports: 1 USB 3.0
Buttons: Reset, WPS
LEDs: Power, System, Wan, Lan 1-4, WiFi 2.4G, WiFi 5G, WPS, USB
Power: DC 12V 1A tip positive
Installation:

Download and flash the manufacturer's built OpenWRT image available at
http://www.cudytech.com/openwrt_software_download
Install the new OpenWRT image via luci (System -> Backup/Flash firmware)
Be sure to NOT keep settings. The force upgrade may need to be checked
due to differences in router naming conventions.

Recovery:

Loads only signed manufacture firmware due to bootloader RSA verification
serve tftp-recovery image as /recovery.bin on 192.168.1.88/24
connect to any lan ethernet port
power on the device while holding the reset button
wait at least 8 seconds before releasing reset button for image to
download

Signed-off-by: Alessio Prescenzo <alessioprescenzo@gmail.com>
[ensure unique wireless MAC, fix GPIO pingroup]
Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 4a8eaa5c7c9235212c4af022c18b2dfbadfe557f)