dt-bindings: omap: Update PRM binding for genpd

The PRM (Power and Reset Module) has a register to enable and disable
the related power domain, so let's update the binding for that.
Signed-off-by: Tony Lindgren <tony@atomide.com>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Cc: devicetree@vger.kernel.org
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>

Documentation/devicetree/bindings/arm/omap/prm-inst.txt

@@ -18,6 +18,7 @@ Required properties:
 		(base address and length)
 
 Optional properties:
+- #power-domain-cells:	Should be 0 if the instance is a power domain provider.
 - #reset-cells:	Should be 1 if the PRM instance in question supports resets.
 
 Example:
@@ -25,5 +26,6 @@ Example:
 prm_dsp2: prm@1b00 {
 	compatible = "ti,dra7-prm-inst", "ti,omap-prm-inst";
 	reg = <0x1b00 0x40>;
+	#power-domain-cells = <0>;
 	#reset-cells = <1>;
 };