regulator: consumer device interface

Add support to allow consumer device drivers to control their regulator
power supply.

This uses a similar API to the kernel clock interface in that consumer
drivers can get and put a regulator (like they can with clocks atm) and
get/set voltage, current limit, mode, enable and disable. This should
allow consumers complete control over their supply voltage and current
limit. This also compiles out if not in use so drivers can be reused in
systems with no regulator based power control.

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