dt-bindings: serial: add generic DT binding for announcing RTS/CTS lines

Add support of generic DT binding for annoucing RTS/CTS lines. The initial
binding 'st,hw-flow-control' is not needed anymore since generic binding
is available, but is kept for backward compatibility.

Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200520133932.30441-2-erwan.leray@st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/st,stm32-uart.yaml

@@ -35,9 +35,11 @@ properties:
     description: label associated with this uart
 
   st,hw-flow-ctrl:
-    description: enable hardware flow control
+    description: enable hardware flow control (deprecated)
     $ref: /schemas/types.yaml#/definitions/flag
 
+  uart-has-rtscts: true
+
   dmas:
     minItems: 1
     maxItems: 2