ath79: add support for I-O DATA ETG3-R

ath79: add support for I-O DATA ETG3-R

I-O DATA ETG3-R is a wired router, based on Atheros AR9342.

Specification:

- Atheros AR9342
- 64 MB of RAM
- 8 MB of Flash
- 5x 10/100/1000 Ethernet
  - AR8327N
- 2x LEDs, 1x key
- UART header on PCB
  - Vcc, GND, TX, RX from ethernet port side
  - 115200n8

Flash instruction using initramfs image:

1. Connect serial cable to UART header on ETG3-R
2. Rename OpenWrt initramfs image to "1500A8C0.img" and place it in
the TFTP directory
3. Set IP address of the computer to 192.168.0.10, connect to the
LAN port of ETG3-R, and start the TFTP server on the computer
4. Connect power cable to ETG3-R and turn on the router
5. Press "Enter" key when the "Hit any key to stop autoboot:" message
is displayed on the console and enter the u-boot cli
6. execute following commands to change kernel address for u-boot
  setenv bootsf 1
  setenv mtd_kernel1 "bootm 0x9f050000"
  saveenv
7. execute "tftpboot; bootm" command to download the initramfs image from
TFTP server and boot with it
8. On the initramfs image, execute "mtd erase firmware" to erase stock
firmware and execute sysupgrade with sysupgrade image for ETG3-R
9. Wait ~150 seconds to complete flashing

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