regulator: extend the fixed dummy voltage regulator to accept voltage

Trivially extend the regulator_register_always_on() helper function to be
even more useful by adding a voltage parameter to it.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>