dt-bindings: Add "mrvl", a legacy vendor prefix for Marvell

While the preferred vendor prefix is "marvell", "mrvl" is used by many
older bindings already. Add it, while also marking it deprecated.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Rob Herring <robh@kernel.org>