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>

Documentation/devicetree/bindings/regulator/regulator.txt

@@ -33,13 +33,16 @@ Optional properties:
   decreases of any level. This is useful for regulators with exponential
   voltage changes.
 - regulator-soft-start: Enable soft start so that voltage ramps slowly
+- regulator-state-standby sub-root node for Standby mode
+  : equivalent with standby Linux sleep state, which provides energy savings
+  with a relatively quick transition back time.
 - regulator-state-mem sub-root node for Suspend-to-RAM mode
   : suspend to memory, the device goes to sleep, but all data stored in memory,
   only some external interrupt can wake the device.
 - regulator-state-disk sub-root node for Suspend-to-DISK mode
   : suspend to disk, this state operates similarly to Suspend-to-RAM,
   but includes a final step of writing memory contents to disk.
-- regulator-state-[mem/disk] node has following common properties:
+- regulator-state-[mem/disk/standby] node has following common properties:
 	- regulator-on-in-suspend: regulator should be on in suspend state.
 	- regulator-off-in-suspend: regulator should be off in suspend state.
 	- regulator-suspend-min-microvolt: minimum voltage may be set in