dt-bindings: imx-gpcv2: Fix the unit address

In the provided example the unit address does not match the 'reg' value,
as IMX7_POWER_DOMAIN_PCIE_PHY is defined as 1.

Fix the unit address and avoid using defines in reg as per Rob
Herring's recommendation.
Signed-off-by: NFabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/power/fsl,imx-gpcv2.txt

@@ -44,10 +44,10 @@ Example:
 			#address-cells = <1>;
 			#size-cells = <0>;
 
-			pgc_pcie_phy: power-domain@3 {
+			pgc_pcie_phy: power-domain@1 {
 				#power-domain-cells = <0>;
 
-				reg = <IMX7_POWER_DOMAIN_PCIE_PHY>;
+				reg = <1>;
 				power-supply = <&reg_1p0d>;
 			};
 		};