ramips: add support for Edimax RA21S

ramips: add support for Edimax RA21S

Edimax RA21S is a dual band 11ac router,
based on MediaTek MT7621A and MT7615N chips.

Specification:
- SoC: MediaTek MT7621A dual-core @ 880MHz
- RAM: 256M (Nanya NT5CC128M16IP)
- FLASH: 16MB (Macronix MX25L12835F)
- WiFi: 2.4/5 GHz 4T4R
  - 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 case closed
- UART:  through-hole on PCB.
   J1: 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://192.168.2.1/)
User: admin
Password: 1234

The sysupgrade image can be installed via TFTP
from the U-Boot bootloader. Connect via ethernet port 2.

Tested on device by @UAb5eSMn

Signed-off-by: Maksym Medvedev <redrathnure@gmail.com>
[split DTS and take over improvements from RG21S, extend commit
message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>