input: sensor: Add power init and enable/disable function for stk3x1x

Power control function is required to request regulator and
configure supply voltage for device.
Power enable/disable function will be called to turn ON/OFF device
power supply.

Change-Id: Ie5dd587fdee06b2dbdf15941468838c4f46933c2
Signed-off-by: Bingzhe Cai <bingzhec@codeaurora.org>