mfd: PCF50633 core driver

This patch implements the core of the PCF50633 driver. This core driver has
generic register read/write functions and does interrupt management for its
sub devices.

Signed-off-by: Balaji Rao <balajirrao@openmoko.org>
Cc: Andy Green <andy@openmoko.com>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>