bindings: perf: imx-ddr: Add new compatible string

Add new compatible string for i.MX8MPlus DDR PMU core.
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Will Deacon <will@kernel.org>

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

@@ -5,6 +5,7 @@ Required properties:
 - compatible: should be one of:
 	"fsl,imx8-ddr-pmu"
 	"fsl,imx8m-ddr-pmu"
+	"fsl,imx8mp-ddr-pmu"
 
 - reg: physical address and size