regulator: anatop: Add power gating support to digital LDOs

The ARM, PU, and SOC LDOs in the i.MX6 PMU can completely gate
their power output. Since power gating is configured by writing
zero to the voltage target bitfield,, store a copy of the
voltage selector to be restored when reenabling the regulator.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>