ARM: mvebu: MPIC: read number of interrupts from control register

Read the number of MPIC interrupts from the controller and only register
that many.

[gregory.clement@free-electrons.com: rename armada symbol name to fit
with new name: armada_370_xp]

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Lior Amsalem <alior@marvell.com>