mn10300: Use the core irq stats function

Let the core do the irq_desc resolution.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: David Howells <dhowells@redhat.com>
Cc: mn10300 <linux-am33-list@redhat.com>
Link: http://lkml.kernel.org/r/20140223212737.751487689@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>