ramips: add support for Asus RT-AC85P

ramips: add support for Asus RT-AC85P

SoC:	MediaTek MT7621AT dual-core @ 880MHz
RAM:	256M (Winbond W632GG6KB-1)
FLASH:	128MB (Macronix MX30LF1G18AC-TI)
WiFi:	- 2.4GHz MediaTek MT7615N bgn
	- 5GHz MediaTek MT7615N nac
Switch: SoC integrated Gigabit Switch (4 x LAN, 1 x WAN)
USB:	1 x USB 3.1 (Gen 1)
BTN:	Reset, WPS
LED:	- Power (blue)
	- 5Ghz (blue)
	- 2.4GHz (blue)
	- Internet (blue)
	- 4x LAN (blue)
	(LAN/WAN leds are not controllable by GPIOs)
UART: 	UART is present as Pads marked J4 on the PCB.
	3.3V - TX - RX - GND / 57600-8N1
	3.3V is the square pad
MAC:	The MAC address on the router-label matches the MAC of
	the 2.4 GHz WiFi.
	LAN and WAN MAC are identical: MAC_LABEL+4
	5 GHz WiFi MAC: also MAC_LABEL+4

Installation
------------
Via U-Boot tftpd:
Switch on device, within 2s press reset button and keep pressed
until power LED starts blinking slowly.
Upload factory image via tftp put, the router's ip is 192.168.1.1
and expects the client on 192.168.1.75.

The images also work on the Asus RT-AC65P models as tested by Gabor.

Signed-off-by: Birger Koblitz <mail@birger-koblitz.de>
Tested-by: Gabor Varga <vargagab@gmail.com>
[fixed Asus -> ASUS in DTS]
Signed-off-by: Petr Štetiar <ynezz@true.cz>