regulator: core: Support for continuous voltage range

Some regulators can set any voltage within the constraints range,
not being limited to specified operating points.

This patch makes it possible to describe such regulator and makes
the regulator_is_supported_voltage() function behave correctly.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>