regulator: core: Provide regmap based voltage_sel operations

regulator: core: Provide regmap based voltage_sel operations

Since the voltage selector operations are intended to directly map a
bitfield in the device register map into regulator API operations the
code for implementing them is usually very standard we can save some
code by providing standard implementations for devices using the regmap
API.

Drivers using regmap can pass their regmap in in the regmap_config
struct, set vsel_reg and vsel_mask in their regulator_desc and then
use regulator_{get,set}_voltage_sel_regmap in their ops. This saves
a small amount of code from each driver.

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