dt-bindings: perf: imx8-ddr: add imx8qxp ddr performance monitor

Add binding documentation for the imx8qxp DDR performance monitor unit.
Signed-off-by: NFrank Li <Frank.Li@nxp.com>
Reviewed-by: NRob Herring <robh@kernel.org>
[will: Removed trailing newline]
Signed-off-by: NWill Deacon <will.deacon@arm.com>

Documentation/devicetree/bindings/perf/fsl-imx-ddr.txt

+* Freescale(NXP) IMX8 DDR performance monitor
+
+Required properties:
+
+- compatible: should be one of:
+	"fsl,imx8-ddr-pmu"
+	"fsl,imx8m-ddr-pmu"
+
+- reg: physical address and size
+
+- interrupts: single interrupt
+	generated by the control block
+
+Example:
+
+	ddr-pmu@5c020000 {
+		compatible = "fsl,imx8-ddr-pmu";
+		reg = <0x5c020000 0x10000>;
+		interrupt-parent = <&gic>;
+		interrupts = <GIC_SPI 131 IRQ_TYPE_LEVEL_HIGH>;
+	};