ARM: 7590/1: /proc/interrupts: limit the display of IPIs to online CPUs only

This is what is done for the regular interrupts in kernel/irqs/proc.c
already, before calling arch_show_interrupts().  Not doing so for the
IPIs causes the column headers not to match with the content whenever
some CPUs are offline.

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