dt-bindings: Add URT UMSH-8596MD-xT panel bindings

Add DT bindings for United Radiant Technology UMSH-8596MD-xT 7.0" WVGA
TFT LCD panels.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>