ARM: dts: msm: Add support for TS virtual buttons

8916 CDP/MTP use Synaptics S3408 touch controller and 8916 QRD uses
Synaptics S3407A touch controller, both of which require virtual key
driver to drive the virtual buttons on touch panel. Support is added
for MENU, HOME, BACK, and SEARCH button.

Change-Id: Ie54a2b0c4d1e79bb5256d4b550d35ead555cc337
Signed-off-by: Himanshu Aggarwal <haggarwa@codeaurora.org>