ARM: mvebu: fix unit address of MPIC nodes

The Device Tree nodes describing the MPIC nodes on Armada 370, 375,
38x and XP had a unit address that did not match the first reg
property, as suggested by the ePAPR. This commit fixes that.

[gregory.clement@free-electrons.com: removed the armada-38x part, as it
was already applied by a previous patch]

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