dt/irq: add empty of_irq_count for !OF_IRQ

Add an empty version of of_irq_count for !OF_IRQ. This fixes build error
on sparc in linux-next:

drivers/gpio/gpio-bcm-kona.c:542: undefined reference to `of_irq_count'

Signed-off-by: Rob Herring <rob.herring@calxeda.com>