dt-bindings: arm: mrvl: amend Browstone compatible string

The Brownstone board is compatible with "mrvl,mmp2". The actual DTS
already contains the string -- add it to the binding doc as well.
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/arm/mrvl/mrvl.txt

@@ -11,4 +11,4 @@ Required root node properties:
 
 MMP2 Brownstone Board
 Required root node properties:
-	- compatible = "mrvl,mmp2-brownstone";
+	- compatible = "mrvl,mmp2-brownstone", "mrvl,mmp2";