serial: mfd: add more baud rates support

Add more baud rates support referring the baud_table[] defined
in drivers/char/tty_ioctl.c: 3000000/2000000/1000000/500000

Signed-off-by: Feng Tang <feng.tang@intel.com>
Acked-by: Alan Cox <alan.cox@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>