ARM: mvebu: do not duplicate the mpic alias

The mpic alias is already defined in the common armada-370-xp.dtsi, so
there's no need to repeat it at the armada-xp.dtsi and armada-370.dtsi
level.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>