dt-bindings: perf: fsl-imx-ddr: Add a compatible string fsl,imx94-ddr-pmu for i.MX94

i.MX94 has a DDR Performance Monitor Unit which is compatible with i.MX93.
This will add a compatible for i.MX94.

Reviewed-by: Peng Fan <[e-mail]>
Reviewed-by: Frank Li <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Signed-off-by: Xu Yang <[e-mail]>
Signed-off-by: Will Deacon <[e-mail]>

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

···
       - items:
           - enum:
               - fsl,imx91-ddr-pmu
+              - fsl,imx94-ddr-pmu
               - fsl,imx95-ddr-pmu
           - const: fsl,imx93-ddr-pmu