regulator: bindings: Add properties for coupled regulators

Some regulators require keeping their voltage spread below defined
max_spread.

Add properties to provide information on regulators' coupling.
Signed-off-by: Maciej Purski <m.purski@samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/regulator.txt

@@ -73,6 +73,11 @@ Optional properties:
 	0: Disable active discharge.
 	1: Enable active discharge.
 	Absence of this property will leave configuration to default.
+- regulator-coupled-with: Regulators with which the regulator
+  is coupled. The linkage is 2-way - all coupled regulators should be linked
+  with each other. A regulator should not be coupled with its supplier.
+- regulator-coupled-max-spread: Max spread between voltages of coupled regulators
+  in microvolts.
 
 Deprecated properties:
 - regulator-compatible: If a regulator chip contains multiple