ramips: add support for Tama W06

ramips: add support for Tama W06

Tama Electric Axing W06 is a 2.4 GHz band 11n router, based on Mediatek
MT7688AN.

Specification:

- MT7688AN (575 MHz)
- 64 MB of RAM (DDR2 SDRAM)
- 16 MB of Flash (SPI)
- 1T1R 2.4 GHz
- 1x 10/100/1000 Mbps Ethernet
- 4x LEDs (GPIO connected: 3), 1x button
- 1x USB 2.0 Type-A (host)
- UART header on PCB (GND, RX, TX, Vcc from RJ45 side)

Flash instruction using sysupgrade image:
1. Connect micro-USB cable for power supply into W06 and turn on the
router
2. Connect to wifi with SSID "tama-*" with password. Complete SSID and
password are listed on the back of the router
3. Access to 192.168.1.1 and login with user name "admin" and password
empty
4. In firmware update（ファームウェア更新） page, click "参照" button
and click "ブラウザー" button to open file browser, select the
sysupgrade image and press OK button
5. Wait ~150 seconds to complete flashing

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