ath79: add support for ELECOM WRC-300GHBK2-I

ath79: add support for ELECOM WRC-300GHBK2-I

ELECOM WRC-300GHBK2-I is a 2.4 GHz wireless router, based on Qualcomm
Atheros QCA9563.

Specification:

- Qualcomm Atheros QCA9563
- 64 MB of RAM (DDR2)
- 8 MB of Flash (SPI-NOR)
- 2T2R 2.4 GHz wifi
  - SoC internal
- 5x 10/100/1000 Mbps Ethernet
- 3x LEDs, 4x keys(connected to GPIO: 3x)
- UART header on PCB
  - TX, GND, RX, Vcc from ethernet port side
  - 115200n8

Flash instruction using factory image:

1. Boot the WRC-300GHBK2-I normaly and connect the computer to its
LAN port
2. Access to "http://192.168.2.1/" and open firmware update page
("ファームウェア更新 手動更新（アップデート）")
3. Select the OpenWrt factory image and click apply ("適用") button
to perform firmware update
4. On the (initramfs) factory image, execute "mtd erase firmware" to
erase stock firmware and execute sysupgrade with squashfs-sysupgrade
image for WRC-300GHBK2-I
5. Wait ~150 seconds to complete flashing

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