ath79: add support for EnGenius EWS511AP

ath79: add support for EnGenius EWS511AP

EnGenius EWS511AP is a wireless managed wall AP with PoE support,
based on Qualcomm/Atheros QCA9531(Honeybee) + QCA9887.

Short specification:

- 128MB of RAM
- 16 MB of SPI FLASH
- 2T2R 2.4 GHz (QCA9531), 802.11b/g/n
- 1T1R 5 GHz (QCA9887), 802.11ac/n/a
- 2x 10/100 Mbps Ethernet (one port with PoE support)
- 1x Power LED, 2x LAN LEDs, 1x WLAN 2.4G LED, 1x WLAN 5G LED
- 1x RESET button
- built-in watchdog chipset

Flash instruction:

From EnGenius firmware to OpenWrt firmware:

Original firmware is based on QSDK.
Use sysupgrade firmware directly in vendor GUI.
Reset to factory default is necessary.

From OpenWrt firmware to EnGenius firmware:

1. Setup a TFTP server on your computer and configure static IP to 192.168.99.8
   Put the OpenWrt firmware in the root directory on your computer.
2. Power up EWS511AP. Press 4 and then press any key to enter u-boot.
3. Download OpenWrt firmware
   (ath)> tftpboot 0x80060000 ${dir}"openwrt-ath79-generic-engenius_ews511ap-squashfs-sysupgrade.bin"
4. Flash the firmware
   (ath)> erase 0x9f060000 +f50000
   (ath)> cp.b $fileaddr 0x9f060000 $filesize
5. Reboot
   (ath)> reset

Signed-off-by: Guan-Hong Lin <GH.Lin@senao.com>