arm: mvebu: fix typo in .dtsi comment for Armada XP SoCs

The comment was wrongly referring to Armada 370 while the file is
related to Armada XP.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Olof Johansson <olof@lixom.net>