ramips: add support for RAVPower RP-WD009

ramips: add support for RAVPower RP-WD009

The RAVPower RP-WD009 is a batter-powered pocket sized router with SD
card lot and USB port.

Hardware
--------
CPU:   MediaTek MT7628AN
RAM:   64M DDR2
FLASH: 16M GigaDevices SPI-NOR
WLAN:  MediaTek MT7628AN 2T2R b/g/n
       MediaTek MT7610E  1T1R n/ac
ETH:   1x FastEthernet
SD:    SD Card slot
USB:   USB 2.0

Custom PMIC on the I2C bus (address 0x0a).

Installation
------------

1. Press and hold down the reset button.

2. Power up the Device. Keep pressing the reset button for 10
   more seconds until the Globe LED lights up.

3. Attach your Computer to the Ethernet port. Assign yourself the
   address 10.10.10.1/24.

4. Access the recovery page at 10.10.10.128 and upload the OpenWrt
   factory image.

5. The flashing will take around 1 minute. The device will reboot
   automatically into OpenWrt.

Signed-off-by: David Bauer <mail@david-bauer.net>