input: twl4030: Keypad scan and input

This allows scanning the twl4030 keypad, storing the result in a 64-byte long
matrix with the twl4030_keypad_scan function.

Detecting a key at a given column and row is made easier with the
twl4030_keypad_key function.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-by: Tom Rini <trini@konsulko.com>