m68k: mac - Add SWIM floppy support

It allows to read data from a floppy, but not to write to, and to eject the
floppy (useful on our Mac without eject button).

Signed-off-by: Laurent Vivier <Laurent@lvivier.info>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>