regulator: Deprecate regulator_can_change_voltage()

All current users of regulator_can_change_voltage() are abusing it,
using it to wrap a call to regulator_set_voltage() on probe without any
alternative handling for fixed voltages.  Drivers should only be using
regulator_set_voltage() if they need to vary voltages at runtime, fixed
voltages should normally be set via machine constraints, and calling
regulator_set_voltage() on a regulator which can't be varied will
succeed if the current voltage is within the range requested so users
shouldn't worry if they have permission to vary normally.

Deprecate the API to try to stop any new users appearing while we fix
the current callers.

Signed-off-by: Mark Brown <broonie@kernel.org>