regulator: anatop: Add bypass support to digital LDOs

The ARM, PU, and SOC LDOs in the i.MX6 PMU can operate
in bypass mode. This allows to use external switching
regulators for cpu voltage scaling.

Since bypass and power gating modes are not configured
with their own bits, but via the voltage target bitfield,
store bypass state to be restored when reenabling the
regulator.

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