Documentation: arm: add Marvell Armada 7K and 8K families

As we are adding support for the Armada 7K and 8K families, this
commit adds them to the Marvell documentation listing all supported
SoCs, together with references to their Product Brief, Homepage and
Device Tree files.

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