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>