regulator: core: Allow specify apply_[reg|bit] for regmap based voltage_sel operations

Some DVM regulators needs to update apply_bit after setting vsel_reg to
initiate voltage change on the output.  This patch adds apply_reg and
apply_bit to struct regulator_desc and update
regulator_set_voltage_sel_regmap() to set apply_bit of apply_reg when
apply_bit is set.

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