ARM: mmp2: DT: be compatible with mrvl,mmp2

There are more boards that can work with mmp2-dt than just Brownstone.
The OLPC XO-1.75 device tree root is compatible with "mrvl,mmp2" only.

The "mrvl,mmp2-brownstone" string is safe to remove: the Brownstone
device tree contains the "mrvl,mmp2" compatible string too.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Olof Johansson <olof@lixom.net>