extcon: arizona: Implement button detection support

As well as identifying accessories the accessory detection hardware on
Arizona class devices can also detect a number of buttons which we should
report via the input API.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>