arm64: dts: imx8qxp: add fallback compatible string for scu pd

According to binding doc, add the fallback compatible string for
scu pd.

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

arch/arm64/boot/dts/freescale/imx8qxp.dtsi

@@ -176,7 +176,7 @@ ocotp: imx8qx-ocotp {
 		};
 
 		pd: imx8qx-pd {
-			compatible = "fsl,imx8qxp-scu-pd";
+			compatible = "fsl,imx8qxp-scu-pd", "fsl,scu-pd";
 			#power-domain-cells = <1>;
 		};