atmel_mxt_ts: Set input property bit to indicate a direct input device

Prop bit is checked in userspace and must be set in
driver in order to be recognized correctly, since it defaults
to mouse pointer.

Change-Id: Id490152174624e86e8b62b746d25baf2d544080a
Signed-off-by: Jessica Gonzalez <jgaona@codeaurora.org>