ARM: mvebu: Fix MPIC unit address

The unit-address is supposed to be equal the first reg address, which is not
the case for the MPIC, that uses the mbus-controller one. Fix this.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>