regulator: regulator test harness

This provides a virtual regulator test harness which exposes a sysfs
interface for setting power requirements, intended for test purposes only.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Liam Girdwood <lg@opensource.wolfsonmicro.com>