regulator: core: Allow regulators to provide a voltage to selector mapping

In order to allow more drivers to factor things out into data allow
drivers to provide a mapping function to convert voltages into selectors.
This allows any driver to use set_voltage_sel(). The existing mapping
based on iterating over list_voltage() is provided as an operation which
can be assigned to the new map_voltage() function though for ease of
transition it is treated as the default.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>