ARM: Show FIQ in /proc/interrupts on CONFIG_FIQ

The show_fiq_list() call in arch/arm/kernel/irq.c currently depends on
CONFIG_ARCH_ACORN, but this is not the only architecture that supports
the usage of FIQ. Change to calling this if CONFIG_FIQ is set (which
is what arch/arm/kernel/fiq.c is built by).

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>