serial: uapi: Add support for bus termination

The Siemens IOT2040 comes with a RS485 interface that allows to enable
or disable bus termination via software. Add a bit to the flags field of
serial_rs485 that applications can set in order to request this feature
from the hardware. This seems generic enough to add it for everyone.
Existing driver will simply ignore it when set.

Signed-off-by: Sascha Weisenberger <sascha.weisenberger@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>