arm64: dts: imx8: add a node label to ddr-pmu

The ddr-pmu on i.mx8dxl has a different interrupt number.
Add a node label to ddr-pmu so that it could be referred
and changed in i.mx8dxl dts.
Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8-ss-ddr.dtsi

@@ -10,7 +10,7 @@ ddr_subsys: bus@5c000000 {
 	#size-cells = <1>;
 	ranges = <0x5c000000 0x0 0x5c000000 0x1000000>;
 
-	ddr-pmu@5c020000 {
+	ddr_pmu0: ddr-pmu@5c020000 {
 		compatible = "fsl,imx8-ddr-pmu";
 		reg = <0x5c020000 0x10000>;
 		interrupts = <GIC_SPI 131 IRQ_TYPE_LEVEL_HIGH>;