[POWERPC] Make endianess of cfg_addr for indirect pci ops runtime

Make it so we do a runtime check to know if we need to write cfg_addr
as big or little endian.  This is needed if we want to allow 86xx support
to co-exist in the same kernel as other 6xx PPCs.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>