ramips: add support for Edimax EW-7478APC

SoC:	MediaTek MT7620a @ 580MHz
RAM:	64M (Winbond W9751G6KB-25)
FLASH:	8MB (Macronix)
WiFi:	SoC-integrated: MediaTek MT7620a bgn
WiFi:	MediaTek MT7612EN nac
Switch: Mediatek MT7530W Gigabit Switch (4 x LAN, 1 x WAN)
USB:	Yes 1 x 2.0 (+ 1 x 2.0 unpopulated header)
BTN:	Reset/WPS
LED:    - Power (white)
	- Internet (blue)
	- Wifi (blue)
	- USB (blue)
UART:  UART is present as Pads with throughholes on the PCB. They are
       located in the lower right corner (GbE ports facing up)
       3.3V - RX - GND - TX / 57600-8N1
       3.3V is the square pad

Installation
------------
Update the factory image via the web-interfaces (by default:
http://edimax.setup

)

Signed-off-by: Birger Koblitz <mail@birger-koblitz.de>
[merge conflicts in 01_leds and mt7620.mk, dts whitespace issues]
Signed-off-by: Petr Štetiar <ynezz@true.cz>