powerpc/irq: Move get_irq() comment into header

The guts of do_IRQ() isn't really the right place to be documenting
the ppc_md.get_irq() interface. So move the comment into machdep.h

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>