dt-bindings: serial: Document CTS/RTS gpios in STM32 UART

dt-bindings: serial: Document CTS/RTS gpios in STM32 UART

Document the use of CTS/RTS gpios for flow control in STM32 UART
controller. These properties can be used instead of 'st,hw-flow-ctrl'
for making use of any gpio pins for flow control instead of dedicated
pins. It should be noted that both CTS/RTS and 'st,hw-flow-ctrl'
properties cannot co-exist in a design.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200420170204.24541-2-mani@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>