regulator: provide a helper for registering a fixed regulator

Some devices require a regulator to work, but boards may not have
a software controllable regulator for this device. Provide a helper
function to make it simpler for these boards to register a fixed
regulator as a dummy regulator.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>