ath79: add support for WD My Net N750

ath79: add support for WD My Net N750

SoC: AR9344
RAM: 128MB
Flash: 16MiB Winbond 25Q128BVFG SPI NOR
5GHz WiFi: AR9380 PCIe 3x3:3 802.11n
2.4GHz WiFi: AR9344 (SoC) AHB 2x2:2 802.11n

5x Gigabit ethernet via AR8327N switch (green + amber LEDs)
2x USB 2.0 via GL850G hub
4x front LEDs from SoC GPIO
1x front WPS button from SoC GPIO
1x bottom reset button from SoC GPIO

Known issues:
AR8327N LEDs only have default functionality, not presented in sysfs.
This is a regression from ar71xx.

UART header JP1, 115200 no parity 1 stop
TX
GND
VCC
(N/P)
RX

See https://openwrt.org/toh/wd/n750 for flashing detail.
Procedures unchanged from ar71xx.

Tested sysupgrade + factory flash from WD Emergency Recovery

Signed-off-by: Ryan Mounce <ryan@mounce.com.au>