sh: Use asm-generic/io.h

Convert the sh architecture to make use of the new asm-generic/io.h to
provide address mapping functions. As the generic implementations are
suitable for sh this is primarily a matter of moving code.

Feedback from architecture maintainers is welcome.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>