This patch adds basic support for the PCF50633 ADC. The subtractive mode
is not supported yet.

Since we don't have adc subsystem, it currently lives in drivers/mfd.
Signed-off-by: NBalaji Rao <balajirrao@openmoko.org>
Cc: Andy Green <andy@openmoko.com>
Acked-by: NJonathan Cameron <jonathan.cameron@gmail.com>
Signed-off-by: NSamuel Ortiz <sameo@openedhand.com>