m68k/io: Switch mmu variant to <asm-generic/io.h>

The dummy functions defined in <asm/io_mm.h> can be provided by
<asm-generic/io.h>.

As nommu already uses <asm-generic/io.h>, move its inclusion to
<asm/io.h>, and add/adjust include guards where appropriate.

This gets rid of lots of "statement with no effect" and "unused
variable" warnings when compile-testing.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>