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

arch/arm/boot/dts/armada-xp.dtsi

@@ -12,7 +12,7 @@
  * License version 2.  This program is licensed "as is" without any
  * warranty of any kind, whether express or implied.
  *
- * Contains definitions specific to the Armada 370 SoC that are not
+ * Contains definitions specific to the Armada XP SoC that are not
  * common to all Armada SoCs.
  */