ramips: add support for Sanlinking D240

The Sanlinking Technologies D240
(http://www.sanlinking.com/en/29-dual-4g-wifi-router.html) is basically the same
device as the ZBT WE826, so adding support for it in LEDE is straight forward.
The differences is that the D240 has two mini-PCIe slots (instead of one), blue
LEDs and supports PoE.

Specification:
* CPU: MT7620A
* 1x 10/100Mbps POE (802.3af/802.3at) Ethernet, 4x 10/100Mbps.
* 16 MB Flash.
* 128 MB RAM.
* 1x USB 2.0 port.
* 2x mini-PCIe slots.
* 2x SIM slots.
* 1x 2.4Ghz WIFI.
* 1x button.

Wifi, USB, switch and both mini-PCIe slots are working. I have not been able to
test the SD card reader.

The device comes pre-installed with an older version of OpenWRT, including Luci.
In order to install LEDE, you need to follow the existing procedure for updating
OpenWRT/LEDE using Luci. I.e., you need to access the UI and update the firmware
using the sysupgrade-image. Remember to select that you do not want to keep
existing settings. The default router address is 192.168.10.1 and
username/password admin/root (at least on my devices).

If you brick the device, the procedure for recovery is the same as for the
WE826. Please see the wiki page for that device for instructions.

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