Documentation: arm: update supported Marvell EBU processors

Now that we support Armada 39x, let's add this family of SoC to the
Marvell documentation, and a reference to a link with more details
about those processors. Unfortunately, no datasheet is publicly
available at this time.
Signed-off-by: NThomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: NGregory CLEMENT <gregory.clement@free-electrons.com>

Documentation/arm/Marvell/README

@@ -96,6 +96,11 @@ EBU Armada family
 	88F6820
 	88F6828
 
+  Armada 390/398 Flavors:
+	88F6920
+	88F6928
+    Product infos: http://www.marvell.com/embedded-processors/armada-39x/
+
   Armada XP Flavors:
         MV78230
         MV78260