tty: serial: meson: Add support for XTAL clock input

Fix the baudrate calculation for 24 MHz XTAL clock found on gxbb platforms.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Acked-by: Carlo Caione <carlo@endlessm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>