ramips: add support for Ravpower WD03

ramips: add support for Ravpower WD03

The RavPower WD03 is a battery powered SD card reader and a USB port.

Specifications:
SOC:     MediaTek MT7620N
BATTERY: 6000mah
WLAN:    802.11bgn
LAN:     1x 10/100 Mbps Ethernet
USB:     1x USB 2.0 (Type-A)
RAM:     PM Tech PMD708416CTR-5CN 32 MB
FLASH:   Holtek HT66F40 - 8 MB Flash
LED:     Power button and 4 leds to indicate power level of the
         battery (could not get control of that)
INPUT:   Power, reset button
OTHER:   USB SD-Card reader with card detect on GPIO#42

Tested and working:
 - Ethernet
 - 2.4 GHz WiFi (Correct MAC-address)
 - installation from tftp
 - OpenWRT sysupgrade (Preserving and non-preserving)
 - LEDs
 - Buttons

Installation:
 - Download the sysupgrade image
 - Place it in the root of a clean TFTP server running on your computer.
 - Rename the image to "kernel" — be sure there is no file extension.
 - Plug the WD03 into your computer via ethernet.
 - Set your computer to use 10.10.10.254 as its IP address.
 - With your WD03 shut down, hold down the power button until the first
   white LED lights up.
 - Push and hold the reset button and release the power button. Continue
   holding the reset button for 30 seconds or until it begins searching
   for files on your TFTP server, whichever comes first.
 - The WD03 (10.10.10.128) will look for your computer at 10.10.10.254
   and install the kernel file. Once it has finished installation of the
   kernel file, it will search for a (nonexistent) rootfs file — when it
   begins searching for this file, shut down the WD03 by holding the
   power button normally.
 - Start up your WD03 normally.

Signed-off-by: Matthias Badaire <mbadaire@gmail.com>