arm64/smp: Use irq_desc_kstat_cpu() in arch_show_interrupts()

The irq descriptor is already there, no need to look it up again.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201210194043.546326568@linutronix.de