ramips: add support for Edimax RG21S

ramips: add support for Edimax RG21S

SoC:	MediaTek MT7621AT dual-core @ 880MHz
RAM:	256M (Nanya NT5CC128M)
FLASH:	16MB (Macronix MX25L12835F)
WiFi:	- 2.4GHz MediaTek MT7615N bgn
	- 5GHz MediaTek MT7615N nac
Switch: SoC integrated Gigabit Switch (4 x LAN, 1 x WAN)
USB:	No
BTN:	Reset, WPS
LED:	4 red LEDs, indistinguishable when casing closed
UART: 	UART is present as Pads marked J1 on the PCB.
	3.3V - RX - GND - TX / 57600-8N1
	3.3V is the square pad

Installation
------------
Update the factory image via the OEM web-interface
(by default:http://192.168.1.1)
The sysupgrade image can be installed via TFTP from
the U-Boot bootloader. Connect ethernet port 2.

Signed-off-by: Birger Koblitz <mail@birger-koblitz.de>
[flash node rename, EDIMAX -> Edimax, complete device model name]
Signed-off-by: Petr Štetiar <ynezz@true.cz>