genirq: improve include files

Move the irq_desc related iterators out of irq.h, into irqnr.h, also
available via interrupt.h.

This way non-genirq (and even non-hardirq) architectures get the
common definitions and iterators.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>