Documentation: arm: marvell: Update link to unrestricted Armada 38x Functional Spec

On Marvell website is documentation accessible without need to register or
fill any other forms.

Signed-off-by: Pali Rohár <pali@kernel.org>
Link: https://lore.kernel.org/r/20210125141529.32357-1-pali@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>