Documentation: arm: update supported Marvell EBU processors

Now that we support Armada 37xx, let's add this family of SoC to the
Marvell documentation, and a reference to a link with more details about
those processors. As for Armda 39x, no datasheet is publicly
available at this time.

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