regulator: Document binding for regulator suspend voltage

This patch builds upon (291d761c regulator: Document binding for
regulator suspend state for PM state) to allow setting the uV in
addition to the state at suspend time.

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>