of/powerpc: Move Powermac irq quirk code into powermac pic driver code

The code that figures out what is wrong with the powermac irq device
tree data belongs with the rest of the powermac irq code.  This patch
moves it out of prom_parse.c and into powermac/pic.c so that it is only
compiled in when actually needed.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>