ramips: mt7621: add support for Cudy X6 v2

ramips: mt7621: add support for Cudy X6 v2

Rename existing device to v1 and create common .dtsi
Difference to v1: 16MB Flash

Specifications:

SoC: MediaTek MT7621
RAM: 256 MB
Flash: 16 MB (SPI NOR, XM25QH128C on my device)
WiFi: MediaTek MT7915E
Switch: 1 WAN, 4 LAN (Gigabit)
Buttons: Reset, WPS
LEDs: Two Power LEDs (blue and red; together they form purple)
Power: DC 12V 1A center positive
Serial: 115200 8N1
        C440 - (3V3 - GND - RX - TX) - C41 | v1 and v2
               (P   - G   - R  - T)        | v2 labels them on the board
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.

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

MAC addresses as verified by OEM firmware:

use   address             source
LAN   f4:a4:54:86:75:a2   label
WAN   f4:a4:54:86:75:a3   label + 1
2g    f4:a4:54:86:75:a2   label
5g    f6:a4:54:b6:75:a2   label + LA-Bit set + 4th oktet increased

The label MAC address is found in bdinfo 0xde00.

Signed-off-by: Felix Baumann <felix.bau@gmx.de>
(cherry picked from commit e38de40f8dd350344407fe5a91e81191f6960804)