regulator: dt-bindings: add regulator-state-standby bindings

For state-mem and state-disk regulators can have
various parameters applied such as enabled/disabled,
current mode, voltage etc.

This patch adds documentation on how to set these parameters
in the device tree for the standby state.

Signed-off-by: Andrei Stefanescu <andrei.stefanescu@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>