dts: bindings: serial: stm32: restore tx-rx-swap property

As part of a previous refactor, this property was removed from the STM32
USART binding. The driver code to support this feature was not changed.
This commit simply adds the property back to the new base .yaml for
device trees which used it.

Tested on a proprietary board using the STM32U5 on USART1.

Signed-off-by: Peter Maxwell Warasila <[email protected]>

Author: warasilapm

dts/bindings/serial/st,stm32-uart-base.yaml

@@ -25,6 +25,12 @@ properties:
         only TX pin is used afterwards and should be configured.
         RX/TX conflicts must be handled on user side.
 
+    tx-rx-swap:
+      type: boolean
+      required: false
+      description:
+        Swap the TX and RX pins. Used in case of a cross wired connection.
+
     tx-invert:
       type: boolean
       required: false