ath79: add partial support for Netgear EX7300v2

ath79: add partial support for Netgear EX7300v2

Hardware
--------
SoC: QCN5502
Flash: 16 MiB
RAM: 128 MiB
Ethernet: 1 gigabit port
Wireless No1: QCN5502 on-chip 2.4GHz 4x4
Wireless No2: QCA9984 pcie 5GHz 4x4
USB: none

Installation
------------
Flash the factory image using the stock web interface or TFTP the
factory image to the bootloader.

What works
----------
- LEDs
- Ethernet port
- 5GHz wifi (QCA9984 pcie)

What doesn't work
-----------------
- 2.4GHz wifi (QCN5502 on-chip)
  (I was not able to make this work, probably because ath9k requires
  some changes to support QCN5502.)

Signed-off-by: Wenli Looi <wlooi@ucalgary.ca>