arm64: dts: imx8mm: update pmu compatible

i.MX8MM features four Cortex-A53 cores, update the compatible
to use more accurate "arm,cortex-a53-pmu"
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mm.dtsi

@@ -192,7 +192,7 @@ psci {
 	};
 
 	pmu {
-		compatible = "arm,armv8-pmuv3";
+		compatible = "arm,cortex-a53-pmu";
 		interrupts = <GIC_PPI 7
 			     (GIC_CPU_MASK_SIMPLE(4) | IRQ_TYPE_LEVEL_HIGH)>;
 		interrupt-affinity = <&A53_0>, <&A53_1>, <&A53_2>, <&A53_3>;