input: touchscreen: maxim_sti: set INPUT_PROP_DIRECT property

Touchscreen is typical direct input device. Set the INPUT_PROP_DIRECT
property to make sure that touch coordinates are mapped correctly.

Change-Id: I398890c111a206baaca499d53e600a9de1d556bd
Signed-off-by: Jing Lin <jinglin@codeaurora.org>