regulator: Convert rc5t583 to set_voltage

Not every regulator driver should implement set_voltage_sel callback.
See commit e8eef82
"regulator: Provide a selector based set_voltage_sel() operation".

For rc5t583, the regulator voltage can be mapped onto selector values with a
simple calculation, thus implement set_voltage is better than set_voltage_sel.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>