sh: irq: Provide an arch_probe_nr_irqs() that wraps the machvec def.

This is just a simple arch_probe_nr_irqs() stub that wraps to the
platform defined number of IRQs. This can be made gradually more
intelligent based on what we can infer from the INTC tables and so on.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>