arm: mvebu: fix address-cells in mpic DT node

There is no need to have a #address-cells property in the MPIC Device
Tree node, and more than that, having it confuses the of_irq_map_raw()
logic, which will be used by the Marvell PCIe driver.

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