arm64: dts: marvell: adjust to the latest mv-xor-v2 DT binding

As suggested by Rob Herring, we should:

 1/ Use a SoC-specific compatible string in addition to the more generic
    one.

 2/ The generic compatible string has been changed from
    "marvell,mv-xor-v2" to "marvell,xor-v2".

We simply reflect the changes made to the Device Tree bindings to the
relevant Marvell 7K/8K Device Tree files.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>