ramips: add support for ASUS RT-AC57U

ramips: add support for ASUS RT-AC57U

SoC:   MediaTek MT7621AT
RAM:   128M (Winbond W631GG6KB-15)
FLASH: 16MB (Spansion S25FL128SA)
WiFi:  MediaTek MT7603EN bgn 2SS
WiFi:  MediaTek MT7612EN nac 2SS
BTN:   Reset - WPS
LED:    - Power
        - LAN {1-4}
        - WAN
        - WiFi 2.4 GHz
        - WiFi 5 GHz
        - USB
UART:  UART is present next to the Power LED.
       TX - RX - GND - 3V3 / 57600-8N1
       3V3 is the nearest one to the Power LED.

Installation
------------
Via TFTP:
1. Set your computers IP-Address to 192.168.1.75.
2. Power up the Router with the Reset button pressed.
3. Release the Reset button after 5 seconds.
4. Upload OpenWRT sysupgrade image via TFTP:
 > tftp -4 -v -m binary 192.168.1.1 -c put <IMAGE>

Via SSH:
Note: User/password for SSH is identical with the one used in the
Web-interface.
1. Complete the initial setup wizard.
2. Activate SSH under "Administration" -> "System".
3. Transfer the OpenWrt sysupgrade image via scp:
 > scp owrt.bin admin@192.168.1.1:/tmp
4. Connect via SSH to the router.
 > ssh admin@192.168.1.1
5. Write the OpenWrt image to flash.
 > mtd-write -i /tmp/owrt.bin -d linux
6. Reboot the router
 > reboot

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 14e0e4f138e35c3e2a15cc3a836c939547ee053b)