Documentation: arm: add link to Armada 38x Functional Spec

The Armada 38x Functional Spec is now available (after registration
unfortunately), so add a link to it.

While at it, fix a typo in the reference to the Armada 38x product
page.

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