ramips: mt7621: add support for Netgear R6800

ramips: mt7621: add support for Netgear R6800

This patch adds support for the Netgear R6800, aka Netgear AC1900 and
R6800-100PES.

Specification:
- SoC: MediaTek MT7621AT (880 MHz)
- Flash: 128 MiB NAND
- RAM: 256 MiB
- Wireless: MediaTek MT7615EN b/g/n , MediaTek MT7615EN an+ac
- LAN speed: 10/100/1000
- LAN ports: 4
- WAN speed: 10/100/1000
- WAN ports: 1
- USB 2.0
- USB 3.0
- Serial baud rate of Bootloader and factory firmware: 57600

Known issues:
- Device has 3 wifi LEDs: Wifi 5Ghz, Wifi 2.4Ghz and Wifi on/off.
  Wifi on/off is not used.

Installation:
- apply factory image via stock web-gui.

Back to stock:
- nmrpflash can be used to recover to the stock Netgear firmware.

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>