Documentation: dt: Tidy up the Marvell related files

Over the last releases we have added more and more Marvell related binding
directly in the arm directory. It's time to have our proper directory
inside it, and move all the files in it.
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>