m68k: Use asm-generic/io.h

Convert the m68k architecture to make use of the new asm-generic/io.h to
provide address mapping functions. As the generic implementations are
suitable for m68k this is primarily a matter of emoving code.

Feedback from architecture maintainers is welcome.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Huan Wang <alison.wang@freescale.com>
Cc: Angelo Dureghello <angelo@sysam.it>
Acked-by: Angelo Dureghello <angelo@sysam.it>
Tested-by: Angelo Dureghello <angelo@sysam.it>