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.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -48,6 +48,12 @@ properties:
     minItems: 1
     maxItems: 2
 
+  cts-gpios:
+    maxItems: 1
+
+  rts-gpios:
+    maxItems: 1
+
   wakeup-source: true
 
   rs485-rts-delay: true
@@ -55,6 +61,14 @@ properties:
   linux,rs485-enabled-at-boot-time: true
   rs485-rx-during-tx: true
 
+if:
+  required:
+    - st,hw-flow-ctrl
+then:
+  properties:
+    cts-gpios: false
+    rts-gpios: false
+
 required:
   - compatible
   - reg