Documentation: arm: update Marvell documentation about Armada 375/38x

Documentation: arm: update Marvell documentation about Armada 375/38x

This commit updates the documentation that describes the various
families of SOCs produced by Marvell, together with the corresponding
available technical documents. It adds Armada 375 and Armada 38x, and
adds a link to the product brief for the already supported Armada 370.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>