ath79: add support for EnGenius ECB1750

ath79: add support for EnGenius ECB1750

Specification:

- Qualcomm Atheros SoC QCA9558
- 720/600/200 MHz (CPU/DDR/AHB)
- 128 MB of RAM (DDR2)
- 16 MB of FLASH (SPI NOR)
- 1x 10/100/1000 Mbps Ethernet
- 3T3R 2.4 GHz (QCA9558 WMAC)
- 3T3R 5.8 Ghz (QCA9880-BR4A, Senao PCE4553AH)

https://fccid.io/A8J-ECB1750

Tested and working:

- lan, wireless, leds, sysupgrade (tftp)

Flash instructions:

1.) tftp recovery

- use a 1GbE switch or direct attached 1GbE link
- setup client ip address 192.168.1.10 and start tftpd
- save "openwrt-ath79-generic-engenius_ecb1750-initramfs-kernel.bin" as "ap.bin" in tfpd root directory
- plugin powercord and hold reset button 10secs.. "ap.bin" will be downloaded and executed
- afterwards login via ssh and do a sysuprade

2.) oem webinterface factory install (not tested)

Use normal webinterface upgrade page und select "openwrt-ath79-generic-engenius_ecb1750-squashfs-factory.bin".

3.) oem webinterface command injection

OEM Firmware already running OpenWrt (Attitude Adjustment 12.09).
Use OEM webinterface and command injection. See wiki for details.

https://openwrt.org/toh/engenius/engenius_ecb1750_1

Signed-off-by: sven friedmann <sf.openwrt@okay.ms>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
[use interrupt-driven "gpio-keys" binding]