ath79: add support for ASUS RT-AC59U / ZenWiFi CD6

ath79: add support for ASUS RT-AC59U / ZenWiFi CD6

ASUS RT-AC59U / RT-AC59U v2 are wi-fi routers with a large number of
alternate names, including RT-AC1200GE, RT-AC1300G PLUS, RT-AC1500UHP,
RT-AC57U v2/v3, RT-AC58U v2/v3, and RT-ACRH12.

ASUS ZenWiFi AC Mini(CD6) is a mesh wifi system. The unit labeled CD6R
is the router, and CD6N is the node.

Hardware:

- SoC: QCN5502
- RAM: 128 MiB
- UART: 115200 baud (labeled on boards)
- Wireless:
  - 2.4GHz: QCN5502 on-chip 4x4 802.11b/g/n
    currently unsupported due to missing support for QCN550x in ath9k
  - 5GHz: QCA9888 pcie 5GHz 2x2 802.11a/n/ac
- Flash: SPI NOR
  - RT-AC59U / CD6N: 16 MiB
  - RT-AC59U v2 / CD6R: 32 MiB
- Ethernet: gigabit
  - RT-AC59U / RT-AC59U v2: 4x LAN 1x WAN
  - CD6R: 3x LAN 1x WAN
  - CD6N: 2x LAN
- USB:
  - RT-AC59U / RT-AC59U v2: 1 port USB 2.0
  - CD6R / CD6N: none

WiFi calibration data contains valid MAC addresses.

The initramfs image is uncompressed because I was unable to boot a
compressed initramfs from memory (gzip or lzma). Booting a compressed
image from flash works fine.

Installation:

To install without opening the case:

- Set your computer IP address to 192.168.1.10/24
- Power up with the Reset button pressed
- Release the Reset button after about 5 seconds or until you see the
  power LED blinking slowly
- Upload OpenWRT factory image via TFTP client to 192.168.1.1

Revert to stock firmware using the same TFTP method.

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