sh: Support I/O space swapping where needed.

This adopts a trimmed down version of the MIPS port mangling interface
limited to the I/O swabbing for platforms that can't use little endian
accessors. For platforms with mixed I/O spaces involving PCI it will
still be necessary to enable byte swapping at the host controller level.
Attention needs to be paid to all of host controller endianness, CPU
endianness, and whether I/O accesses are explicitly swapped or not via
SWAP_IO_SPACE. Fortunately the platforms that need this are in the
minority.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>