MIPS: cleanup fixup_bigphys_addr handling

fixup_bigphys_addr is only provided by the alchemy platform.  Remove
all the stubs, and ensure we only call it if it is actually implemented.

Also don't bother implementing io_remap_pfn_range if we don't have to,
and move the remaining implementation to alchemy platform code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>