Documentation: arm: update supported Marvell EBU processors

Now that we support Armada 39x, let's add this family of SoC to the
Marvell documentation, and a reference to a link with more details
about those processors. Unfortunately, no datasheet is publicly
available at this time.

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