m68k: enable binfmt_flat on systems with an MMU

Now that the generic changes are in place, this can be enabled on m68k
with the use of proper user space accessors in the flat_get_addr_from_rp()
and flat_put_addr_at_rp() handlers as rp actually holds a user space
address.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>