dt-bindings: serial: Add rx-tx-swap to stm32-usart

Add new rx-tx-swap property to allow for RX & TX pin swapping.
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: Martin Devera <devik@eaxlabs.cz>
Link: https://lore.kernel.org/r/20210328154306.22674-1-devik@eaxlabs.czSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -9,9 +9,6 @@ maintainers:
 
 title: STMicroelectronics STM32 USART bindings
 
-allOf:
-  - $ref: rs485.yaml
-
 properties:
   compatible:
     enum:
@@ -40,6 +37,8 @@ properties:
 
   uart-has-rtscts: true
 
+  rx-tx-swap: true
+
   dmas:
     minItems: 1
     maxItems: 2
@@ -66,13 +65,23 @@ 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
+allOf:
+  - $ref: rs485.yaml#
+  - $ref: serial.yaml#
+  - if:
+      required:
+        - st,hw-flow-ctrl
+    then:
+      properties:
+        cts-gpios: false
+        rts-gpios: false
+  - if:
+      properties:
+        compatible:
+          const: st,stm32-uart
+    then:
+      properties:
+        rx-tx-swap: false
 
 required:
   - compatible