dt-bindings: arm: mrvl: Document MMP3 compatible string

Marvel MMP3 is a successor to MMP2, containing similar peripherals with two
PJ4B cores.

Signed-off-by: Lubomir Rintel <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/arm/mrvl/mrvl.yaml

···
           - enum:
               - mrvl,mmp2-brownstone
           - const: mrvl,mmp2
+      - description: MMP3 based boards
+        items:
+          - const: mrvl,mmp3
 ...