ath79: Add support for Ubiquiti LiteAP ac (LAP-120)

ath79: Add support for Ubiquiti LiteAP ac (LAP-120)

This commit adds support for the Ubiquiti LiteAP ac (LAP-120), an outdoor
5 GHz AC access point with an integrated 120° 16 dBi antenna. The device
was previously known as LiteBeam AP ac but was later rebranded.

CPU:    Atheros AR9342 SoC
RAM:    64 MB DDR2
Flash:  16 MB NOR SPI
Ports:  1 GbE port (PoE in)
WLAN:   5 GHz QCA899X (PCI)

The integrated QCA899X is a Ubiquiti branded part with modified vendor
and product id (0777:11ac9). It is very similar to the NanoStation loco
AC, except for the 2.4 GHz management radio (missing here).

Installation procedure is the same as the NanoStation [loco] AC:

1. Connect to serial header on device
2. Power on device and enter uboot console
3. Set up tftp server serving an openwrt initramfs build
4. Load initramfs build using the command tftpboot in the uboot cli
5. Boot the loaded image using the command bootm
6. Copy squashfs openwrt sysupgrade build to the booted device
7. Use mtd to write sysupgrade to partition "firmware"
8. Reboot and enjoy

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>