of/pci: move of_irq_map_pci() into generic code

There is a tiny difference between PPC32 and PPC64. Microblaze uses the
PPC32 variant.

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
[grant.likely@secretlab.ca: Added comment to #endif, moved documentation
	block to function implementation, fixed for non ppc and microblaze
	compiles]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>