regulator: Document binding for regulator suspend state for PM state

This patch add regulator suspend state to constraint in dt file. The regulation_
constraints structure already has regulator suspend state field as following.
The regulator suspend state control the state of regulator according to
PM (Power Management) state.
- struct regulator_state state_disk
- struct regulator_state state_mem

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/regulator.txt

@@ -19,6 +19,15 @@ Optional properties:
   design requires. This property describes the total system ramp time
   required due to the combination of internal ramping of the regulator itself,
   and board design issues such as trace capacitance and load on the supply.
+- 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-on-in-suspend: regulator should be on in suspend state.
+	- regulator-off-in-suspend: regulator should be off in suspend state.
 
 Deprecated properties:
 - regulator-compatible: If a regulator chip contains multiple
@@ -34,6 +43,10 @@ Example:
 		regulator-max-microvolt = <2500000>;
 		regulator-always-on;
 		vin-supply = <&vin>;
+
+		regulator-state-mem {
+			regulator-on-in-suspend;
+		};
 	};
 
 Regulator Consumers: