dt-bindings: xilinx-uartps: Add support for cts-override

Flow control is configurable in xilinx-uartps
Add a dt binding to check for the same.

Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>