regulator: of: Add support for parsing microvolts for suspend state

Leverage all the work that was done in (40e20d68 regulator: of: Add
support for parsing regulator_state for suspend state) and throw in
the ability to set suspend microvolts from the device tree.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Chris Zhong <zyw@rock-chips.com>
Signed-off-by: Mark Brown <broonie@kernel.org>