ARM: SMP: collect IPI and local timer IRQs for /proc/stat

The IPI and local timer interrupts weren't being properly accounted
for in /proc/stat.  Collect them from the irq_stat structure, and
return their sum.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>