Documentation: arm: improve Armada 37xx description

In preparation to the introduction of other SoCs in the ARMv8 Armada
EBU family, this commit tweaks the existing description of Armada 37xx
by making the core, homepage and other informations be visible "under"
the Armada 37xx item. Indeed, the new SoCs will not share the same
core or homepage.

In addition, a link to the Product Brief is added.

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