tty: serial: xilinx_uartps: Supply description for missing member 'cts_override'

tty: serial: xilinx_uartps: Supply description for missing member 'cts_override'

Fixes the following W=1 kernel build warning(s):

 drivers/tty/serial/xilinx_uartps.c:205: warning: Function parameter or member 'cts_override' not described in 'cdns_uart'

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: linux-serial@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201104193549.4026187-23-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>