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 <madmaxwell@soundcomesout.com>