ramips: add support for the HNET C108

ramips: add support for the HNET C108

The HNET C108
(http://www.szhwtech88.com/Product-product-cid-100-id-4374.html) is a
mifi based on MT7602A, which has the following specifications:

* CPU: MT7620A
* 1x 10/100Mbps Ethernet.
* 16 MB Flash.
* 64 MB RAM.
* 1x USB 2.0 port. Only power is connected, this port is meant for
charging other devices.
* 1x mini-PCIe slots.
* 1x SIM slots.
* 1x 2.4Ghz WIFI.
* 1x button.
* 6000 mAh battery.
* 5x controllable LEDs.

Works:
* Wifi.
* Switch.
* mini-PCIe slot. Only tested with a USB device (a modem).
* SIM slot.
* Sysupgrade.
* Button (reset).

Not working (also applies to the factory firmware):
* Wifi LED. It is always switched on, there is no relation to the
up/down state or activity of the wireless interface.

Not tested:
* SD card reader.

Notes:
* The C108 has no dedicated status LED. I therefore set the LAN LED as
status LED.

Installation:
The router comes pre-installed with OpenWRT, including a variant of
Luci. The initial firmware install can be done through this UI,
following normal procedure. I.e., access the UI and update the firmware
using the sysupgrade-image. Remember to select that you do not want to
keep existing settings.

Recovery:
If you brick the device, the C108 supports recovery using TFTP. Keep the
reset button pressed for ~5sec when booting to trigger TFTP. Set the
address of the network interface on your machine to 10.10.10.3/24, and
rename your image file to Kernal.bin.

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>