[ARM] pxa: fix redefinition of NR_IRQS

arch/arm/mach-pxa/include/mach/irqs.h:193:1: warning: "NR_IRQS" redefined
arch/arm/mach-pxa/include/mach/irqs.h:263:1: warning: this is the location of the previous definition

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