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 <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

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

···
     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