ARM: orion5x: update intc device tree node to new reg layout

Future irqchip driver for Orion SoCs has a modified reg layout starting
from CAUSE register. The former DT irq driver stub just used what the
non-DT driver wants. Currently, orion5x is not ready for irqchip driver
because of legacy timer and missing DT clk provider. To ease future
conversion and just because orion-intc binding documentation already
reflects new reg layout, we convert orion5x DT now.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>