This driver will subsume the old button-lradc driver and support far more
options. It can sense LRADC channels, PSWITCH, GPIOs and it handles special
"buttons" like headphone insertion and hold detection. It also provides a
more natural description of the buttons using a target-defined table with some
macros to make it easy to read and write. It uniformely handles debouncing on
LRADC channels and PSWITCH.
Change-Id: Ie61d1f593fdcf3bd456ba1d53a1fd784286834ce
