ARM: dts: apq: Add support for virtual buttons

8084 uses Synapics S3408 touch controller. It requires virtual key
driver to drive the four virtual buttons on touh panel. MENU button
so far is not responding properly to touch but HOME, BACK, and SEARCH
buttons work as expected.

Change-Id: I46401bbcabe093a16399c1d6620342e69103c2e4
Signed-off-by: Chun Zhang <chunz@codeaurora.org>