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>

target/linux/ramips/mt7621/base-files/etc/board.d/02_network

@@ -87,6 +87,7 @@ ramips_setup_interfaces()
 		ucidef_add_switch "switch0" \
 			"0:lan:4" "1:lan:3" "2:lan:2" "3:lan:1" "4:wan:5" "6@eth0"
 		;;
+	edimax,ra21s|\
 	edimax,rg21s)
 		ucidef_add_switch "switch0" \
 			"1:lan:4" "2:lan:3" "3:lan:2" "4:lan:1" "0:wan" "6@eth0"
@@ -198,6 +199,7 @@ ramips_setup_macs()
 		wan_mac=$(mtd_get_mac_ascii factory wanmac)
 		label_mac=$(mtd_get_mac_binary radio 0x4)
 		;;
+	edimax,ra21s|\
 	edimax,rg21s)
 		lan_mac=$(mtd_get_mac_ascii u-boot-env ethaddr)
 		wan_mac=$(mtd_get_mac_ascii u-boot-env wanaddr)