ath79: add support for ROSINSON WR818 board

ath79: add support for ROSINSON WR818 board

This commit adds support for the ROSINSON WR818 WiFi-Router

SoC:    Qualcomm Atheros QCA9563,
FLASH:  Winbond W25Q128FV 16MBytes,
WiFi:   QCA9563 b/g/n 3x3 450Mbit/s,
USB:    1x USB 2.0 Type A, 1x USB2.0 Type C,
IN:     WPS/Reset button GPIO1,
OUT:    Power LED red, Internet LED red, WLAN LED red,
        LAN1 LED red, LAN2 LED red, System LED red,
UART:   RX-GPIO18, TX-GPIO22,

Tested and working:
    - Ethernet (LAN + WAN)
    - WiFi
    - OpenWRT sysupgrade
    - Button
    - LEDs

Installation of OpenWRT from vendor firmware:
    - Connect to the Web-interface at http://192.168.1.1
    - Go to "Administration" -> "Firmware Upgrade"
    - Upload the OpenWrt sysupgrade image

Signed-off-by: Rosy Song <rosysong@rosinson.com>