regulator: Document binding for initial and suspend modes

Some regulators can run on different operating modes (opmodes). This
allows systems to choose the most efficient opmode for each regulator.

This patch builds on top of (291d761 regulator: Document binding for
regulator suspend state for PM state) adding a regulator-initial-mode
DT property to configure at startup the operating mode for regulators
that support changing its mode during normal operation and a property
regulator-mode to be used in the regulator-state-[mem/disk] nodes for
regulators that supports changing its operating mode when the system
enters in a suspend state.

The set of possible modes that a regulator can operate depends on the
hardware capabilities so a list of generic operating modes can't be
provided. Instead, each hardware binding should define the list of
valid operating modes for the regulators found on that device.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Mark Brown <broonie@kernel.org>