dt-bindings: imx-gpc: correct the DOMAIN_INDEX using

Actually DOMAIN_INDEX is not used by the client devices to refer to
the power domain, it uses phandle. Corrent the binding doc a bit
to avoid confusing.
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/power/fsl,imx-gpc.txt

@@ -20,8 +20,7 @@ subnodes of the power gating controller 'pgc' node of the GPC and should
 contain the following:
 
 Required properties:
-- reg: the DOMAIN_INDEX as used by the client devices to refer to this
-  power domain
+- reg: Must contain the DOMAIN_INDEX of this power domain
   The following DOMAIN_INDEX values are valid for i.MX6Q:
   ARM_DOMAIN     0
   PU_DOMAIN      1
@@ -54,6 +53,7 @@ Example:
 				reg = <0>;
 				#power-domain-cells = <0>;
 			};
+
 			pd_pu: power-domain@1 {
 				reg = <1>;
 				#power-domain-cells = <0>;