ath79: add support for Netgear EX6400 and EX7300

ath79: add support for Netgear EX6400 and EX7300

This is sold as a dual-band 802.11ac range extender. It has a sliding
switch for Extender mode or Access Point mode, a WPS button, a recessed
Reset button, a hard-power button, and a multitude of LED's, some
multiplexed via an NXP 74AHC164D chip. The internal serial header pinout is
Vcc, Tx, Rx, GND, with GND closest to the corner of the board. You may
connect at 115200 bps, 8 data bits, no parity, 1 stop bit.

Specification:
- System-On-Chip: QCA9558
- CPU/Speed: 720 MHz
- Flash-Chip: Winbond 25Q128FVSG
- Flash size: 16 MiB
- RAM: 128 MiB
- Wireless No1: QCA9558 on-chip 2.4GHz 802.11bgn, 3x3
- Wireless No2: QCA99x0 chip 5GHz 802.11an+ac, 4x4
- PHY: Atheros AR8035-A

Installation:
If you can get to the stock firmware's firmware upgrade option, just feed
it the factory.img and boot as usual. As an alternative, TFTP the
factory.img to the bootloader.

Signed-off-by: Daniel Gimpelevich <daniel@gimpelevich.san-francisco.ca.us>
[whitespace fix in DTS and reorder of make variables]
Signed-off-by: Petr Štetiar <ynezz@true.cz>