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.cz


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