serial: Add Darwin/macOS support for custom and >230400 baudrates

This change is based on the patch proposed by Denis Ahrens in
https://review.coreboot.org/c/flashrom/+/67822

Change-Id: I3e6b88d2b4c2a130b16456752681fd9f807bf6f0
Signed-off-by: Peter Stuge <peter@stuge.se>
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/70571
Original-Reviewed-by: Thomas Heijligen <src@posteo.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/73480
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
4 files changed