dt-bindings: serial: stm32: add RS485 optional properties

Add options for enabling RS485 hardware control and configuring
Driver Enable signal:
- rs485-rts-delay
- rs485-rx-during-tx
- rs485-rts-active-low
- linux,rs485-enabled-at-boot-time

Signed-off-by: Bich Hemon <bich.hemon@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/st,stm32-usart.txt

@@ -15,6 +15,8 @@ Required properties:
 Optional properties:
 - pinctrl: The reference on the pins configuration
 - st,hw-flow-ctrl: bool flag to enable hardware flow control.
+- rs485-rts-delay, rs485-rx-during-tx, rs485-rts-active-low,
+  linux,rs485-enabled-at-boot-time: see rs485.txt.
 - dmas: phandle(s) to DMA controller node(s). Refer to stm32-dma.txt
 - dma-names: "rx" and/or "tx"