ramips: add support for Zorlik ZL5900V2

ramips: add support for Zorlik ZL5900V2

The Zorlik ZL5900V2 is an unbranded clone of HAME MPR-A1/2.  It is
marketed as "3G Wi-Fi Router".  Only the PCB has the model name
"ZL5900V2" printed on it.

Specifications:
- Ralink RT5350F (360 MHz)
- 32 MB RAM
- 8 MB Flash
- 802.11bgn 1T1R
- 1x 10/100 Mbps Ethernet
- 1x USB 2.0 (Type-A)
- 5200 mAh battery

The ramdisk image (not the squashfs sysupgrade image) can be flashed
through the web interface (named "GoAhead") of the factory firmware.
However, as the factory firmware does not cleanly unmount the rootfs
before flashing, the device may hang instead of rebooting after
successful write.  Power cycling the device gets you in OpenWrt where
the squashfs image may be flashed through normal sysupgrade procedure.

Signed-off-by: Vianney le Clément de Saint-Marcq <code@quartic.eu>