serdev: Add method to assert break signal over tty UART port

Adds serdev_device_break_ctl() and an implementation for ttyport.
This function simply calls the break_ctl in tty layer, which can
assert a break signal over UART-TX line, if the tty and the
underlying platform and UART peripheral supports this operation.

Signed-off-by: Neeraj Sanjay Kale <neeraj.sanjaykale@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>