Serial `add_client` can't add write-only clients

[wom-bat]

I'm trying to set up a system where many PDs have (shared) write access to the serial port for debugging, but one can read from the serial port.

This currently seems to be impossible to specify.

    serial_system = Sddf.Serial(sdf, serial_node, serial_driver, serial_virt_tx,
                                serial_virt_rx)

creates a read-write serial system for everyone.

serial_system.add_client()

always adds both Rx and Tx paths.