m68k/serial: Remove references to obsolete serial config options

  - ATARI_MFPSER, ATARI_MIDI, MULTIFACE_III_TTY, and DN_SERIAL
    have no corresponding drivers (anymore),
  - Clean up SERIAL_CONSOLE dependencies and help text.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>