Auto-configure the maximum number of serial ports based on how many can
be present on the architecture:
  - 3 on H8/300,
  - 10 on SuperH,
  - 18 on Reneas ARM.

The default can still be overridden if CONFIG_EXPERT is enabled.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>