dt-bindings: fsl: scu: add fallback compatible string for power domain

SCU power domain can be used in the same way by IMX8QXP and IMX8QM SoCs.
Let's add a "fsl,scu-pd" fallback compatible string to allow other SoCs
to reuse the common part.

Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/arm/freescale/fsl,scu.txt

@@ -58,7 +58,10 @@ This binding for the SCU power domain providers uses the generic power
 domain binding[2].
 
 Required properties:
-- compatible:		Should be "fsl,imx8qxp-scu-pd".
+- compatible:		Should be one of:
+			  "fsl,imx8qxp-scu-pd"
+			followed by "fsl,scu-pd"
+
 - #power-domain-cells:	Must be 1. Contains the Resource ID used by
 			SCU commands.
 			See detailed Resource ID list from:
@@ -154,7 +157,7 @@ firmware {
 		};
 
 		pd: imx8qx-pd {
-			compatible = "fsl,imx8qxp-scu-pd";
+			compatible = "fsl,imx8qxp-scu-pd", "fsl,scu-pd";
 			#power-domain-cells = <1>;
 		};