ramips: Add device DLINK DWR-512-B

The Dlink DWR-512-B modem is a ralink 5350 processor based embedding
a 3G mini-pcie router.

The oem JBOOT bootloader has to be replaced by a RT5350 SDK
U-Boot such as https://github.com/stevenylai/ralink_sdk - U-Boot
configured for the RT5350 256MiB SDR.

Main reason to change the bootloader is the encrypted header used to
store the kernel image. In this way an image can only be generated
using the propietary binboy tool (included in the GPL distribution
from Dlink). The binboy tool doesn't allow to modify the kernel/rootfs
partition scheme. This is considered a big constraint (limited kernel
size and inefficient usage of flash space).

For interested people I pubblished the details of my investigation
about the encrypted firmware header here:
http://lists.infradead.org/pipermail/lede-dev/2016-October/003435.html



Signed-off-by: Giuseppe Lippolis <giu.lippolis@gmail.com>