ath79: add support for NEC Aterm WG800HP

ath79: add support for NEC Aterm WG800HP

NEC Aterm WG800HP is a 2.4/5 GHz band 11ac router, based on Qualcomm
Atheros QCA9563.

Specification:

- Qualcomm Atheros QCA9563
- 64 MB of RAM (DDR2)
- 8 MB of Flash (SPI-NOR)
- 2.4/5 GHz wifi
  - 2.4 GHz: 2T2R (SoC internal)
  - 5 GHz: 1T1R (QCA9887)
- 4x 10/100/1000 Mbps Ethernet
- 8x LEDs, 3x keys (2x buttons, 1x slide-switch)
- UART through-hole on PCB (J2)
  - Vcc, GND, NC, TX, RX from SoC side
  - 115200n8

Flash instruction using factory image:

1. Connect the computer to the LAN port on WG800HP
2. Connect power cable to WG800HP and turn on it
3. Access to "http://192.168.10.1/" and open firmware update page
("ファームウェア更新")
4. Select the OpenWrt factory image and click update ("更新") button
5. Wait ~150 seconds to complete flashing

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>