mfd: PCF50633 adc driver

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: Balaji Rao <balajirrao@openmoko.org>
Cc: Andy Green <andy@openmoko.com>
Acked-by: Jonathan Cameron <jonathan.cameron@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>