[POWERPC] add virq_to_hw accessor routine

This adds an accessor routine virq_to_hw() to the
virq routines which hides the implementation details
of the virq to hwirq map.

Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>