xen: remove nr_irqs_gsi from generic code

xen: remove nr_irqs_gsi from generic code

The concept is X86 specific.

AFAICT the generic concept here is the number of static physical IRQs
which the current hardware has, so call this nr_static_irqs.

Also using "defined NR_IRQS" as a standin for x86 might have made
sense at one point but its just cleaner to push the necessary
definitions into asm/irq.h.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>