ARM: plat-orion: add reg offset to DT irq driver stub

With irqchip driver for Orion SoCs, reg layout of orion-intc has changed.
This updates irq driver stub implemented before to the new reg layout by
adding an offset to the base address passed by DT node. As orion5x still
uses this stub, it cannot be removed yet.

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