regulators: add regulator_can_change_voltage() function

Introduce a regulator_can_change_voltage() function for the subsytems or
drivers which might check if applying voltage change is possible and use
special workaround code when the driver is used with fixed regulators or
regulators with disabled ability to change the voltage.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>