m68k: put definition guards around virt_to_phys and phys_to_virt

The non-MMU and ColdFire IO access functions will be moving to using the
asm-generic/io.h in the near future. To make that possible we need define
guards around the m68k specific virt_to_phys() and phys_to_virt()
functions.

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