arm/dt: add command mode panel dtsi for Truly 720p panel.

The Truly 720p panel can also function in the command mode
since it has on-board memory. Add the required configurations
for supporting the Truly 720p panel in command mode.

Change-Id: I66e2954e3996d737d5118a6b4cc37f602203ac83
Signed-off-by: Ray Zhang <rayzhang@codeaurora.org>
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>