nios2: Do not include linux/irqdomain.h from asm/irq.h

Including linux/irqdomain.h from asm/irq.h is going to break
as soon as linux/irqdomain.h will include linux/irq.h, so
let's fix this. Code relying on linux/irqomain.h should include
it directly.

Signed-off-by: Marc Zyngier <maz@kernel.org>