[POWERPC] Abolish iopa(), mm_ptov(), io_block_mapping() from arch/powerpc

These old-fashioned IO mapping functions no longer have any callers in
code which remains relevant on arch/powerpc.  Therefore, this removes
them from arch/powerpc.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>