powerpc: Use asm-generic/io.h

Convert the powerpc architecture to make use of the new asm-generic/io.h
to provide address mapping functions. As powerpc can actually perform
non-identity mapping between physical & virtual addresses we can't
simply make use of the generic phys_to_virt() & virt_to_phys()
functions. However since map_physmem() already effectively implemented
the same thing as virt_to_phys() we can simply implement virt_to_phys()
instead of map_physmem() & use the generic map_physmem(). We also drop
the no-op unmap_physmem().

This has only been build-tested, feedback from architecture maintainers
is welcome.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Wolfgang Denk <wd@denx.de>