ARM: get rid of asm/irq.h in asm/prom.h

Avoid potential build problems caused by lacking mach/irqs.h includes
on non-OF builds caused by an errant include in asm/prom.h.  asm/prom.h
requires nothing from asm/irq.h, as Grant says:

On Mon, Feb 06, 2012 at 05:56:23AM +0000, Grant Likely wrote:
> On Sat, Feb 04, 2012 at 10:17:48PM +0000, Russell King wrote:
> > Finally, do we need asm/irq.h in our asm/prom.h ?  That's causing
> > fragility between DT and non-DT builds, because people are finding
> > that their DT builds work without their mach/irqs.h includes but
> > fail when built with non-DT.  The only thing which DT might need -
> > at the most - is NR_IRQS, but I'd hope with things like irq domains
> > it doesn't actually require it.
>
> I don't think so.  There may be a file or two that break because they're
> not including everything they need, but I don't think anything in the
> header requires it.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>