m68k: use asm-generic/io.h for non-MMU io access functions

There is nothing really special about the non-MMU m68k IO access functions.
So we can easily switch to using the asm-generic/io.h functions.

The only thing we do need to handle is that historically the m68k IO access
functions for readw/readl/writew/writel use native CPU endian ordering. So
for us on m68k/ColdFire that means they are big-endian. Leave the existing
set of _raw_read/__raw_write and read/write macros in place to deal with
them. (They are ripe for later cleanup, but that is for another patch).

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