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: NPali Rohár <pali@kernel.org>
Link: https://lore.kernel.org/r/20210125141529.32357-1-pali@kernel.orgSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/arm/marvell.rst

@@ -127,7 +127,7 @@ EBU Armada family
 	- 88F6828 Armada 388
 
     - Product infos:   http://www.marvell.com/embedded-processors/armada-38x/
-    - Functional Spec: https://marvellcorp.wufoo.com/forms/marvell-armada-38x-functional-specifications/
+    - Functional Spec: http://www.marvell.com/content/dam/marvell/en/public-collateral/embedded-processors/marvell-embedded-processors-armada-38x-functional-specifications-2015-11.pdf
 
   Core:
 	ARM Cortex-A9