arm: mvebu: fix compatible string in armada-370-xp.dtsi

All the Device Tree files for Armada 370 and XP SoCs and boards use
the "armada-370-xp" common compatible string, except
armada-370-xp.dtsi which was specifying "armada_370_xp".

Fix this inconsistency by making armada-370-xp.dtsi declare a
compatible string of "armada-370-xp" like everyone else.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>