regulator: core: Support fixed voltages in regulator_is_supported_voltage()

Currently regulator_is_supported_voltage() works by enumerating the set
of voltages which can be set by the regulator but the checks we're doing
to impose constraints mean that if we can't vary the voltage we'll not
report any voltages as supported even though the regulator is actually
set at that voltage.

We could fix the voltage listing but this would mean that list_voltage()
could end up going to the hardware to get the current voltage which isn't
expected (it's supposed to be very cheap) so instead special case things
when we can't change the voltage and compare the requested range against
the current voltage.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>