Introduce helper functions to configure power and interrupt registers.
Default to IDLE mode on probe as device supports auto wakeup to ACVIE mode
on detecting finger touch.

Configure interrupt mode and polarity on start up.  Power down on device
closure or module removal.
Signed-off-by: Roger Quadros <rogerq@ti.com>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>