Input: add support for LM8333 keypads

This driver adds support for the keypad part of the LM8333 and is
prepared for possible GPIO/PWM drivers. Note that this is not a MFD
because you cannot disable the keypad functionality which, thus,
has to be handled by the core anyhow.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>