arm64: dts: ls1088a: add missing PMU node

The Performance Manager Unit was not described in the DTS
which meant performance event monitoring was not possible.

This was exposed by a change to the PMU handling in KVM
in 5.11-rc3 which now prevents a PMU being exposed to a
guest when the host does not provide one:
"KVM: arm64: Don't access PMCR_EL0 when no PMU is available"

Signed-off-by: Mathew McBride <matt@traverse.com.au>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/fsl-ls1088a.dtsi

@@ -189,6 +189,11 @@
 			     <1 10 IRQ_TYPE_LEVEL_LOW>;/* Hypervisor PPI */
 	};
 
+	pmu {
+		compatible = "arm,cortex-a53-pmu";
+		interrupts = <GIC_PPI 7 IRQ_TYPE_LEVEL_LOW>;
+	};
+
 	psci {
 		compatible = "arm,psci-0.2";
 		method = "smc";