ramips: adding support for Asus RP-AC56

ramips: adding support for Asus RP-AC56

Specifications:
	CPU:     MT7621A dual-core 880MHz
	RAM:     64MB DDR2
	FLASH:   16MB MX25L12805D NOR SPI
	WIFI:    2.4GHz 2x2 MT7603 b/g/n PCI
	WIFI:    5GHz 2x2 MT7662 a/b/ac PCI
	ETH:     1xLAN 1000base-T integrated
	SWITCH:  MT7530 Port 0: LAN, Port 6: CPU
	LED:     Power, 2.4GHz WiFi, 5GHz WiFi
	BTN:     WPS, Reset
	UART:    Near ETH port, from ETH: 3V3-TxD-GND-RxD 57600 8n1
	MISC:    Audio support

Installation:
	1. Update using recovery mode
	   - while holdig "reset" button, power on the device
	   - keep holding "reset" until power led is flashing yellow
	   - set own IP to 192.168.1.75, subnet mask: 255.255.255.0
	   - push firmware image (can be factory.bin or sysupgrade.bin)
	     using tftp client in binary mode to 192.168.1.1

Notes:
	This board has only two MAC addresses programmed in the "factory" partition:
	- MAC for wlan0 (2.4GHz) at offset 0x0004
	- MAC for wlan1 (5GHz) at offset 0x8004
	- stock firmware re-uses wlan0 MAC for ethernet
	- no valid addresses found in 0x28, 0x2e, 0xe000 and 0xe006

Signed-off-by: Lea Teuberth <lea.teuberth@outlook.com>