Regulator: Implement list_voltage for pcf50633 regulator driver.

This patch implements list_voltage for the pcf50644 regulator driver.
As the voltages are linearly scaled the code to convert register values to
voltages can be reused and most of the code can be shared with get_voltage.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>