m68k: group io mapping definitions and functions

Create a new header file, kmap.h, that groups all the definitions and
functions associated with the io mapping and remapping.

Currently the functions are spread across raw_io.h and io_mm.h. And in
the future we will want to use these in io_no.h as well. So it makes
sense to move them all together into a single header file.

It is named after the arch/m68k/mm/kmap.c file that actually implements
many of the exported functions.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>
Tested-by: Angelo Dureghello <angelo@sysam.it>