[POWERPC] Uninline and export virq_to_hw()

Uninline virq_to_hw and export it so modules can use it.  The alternative
would be to export the irq_map array instead, but it's an infrequently
called function, and keeping the array unexported seems considerably
cleaner.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>