tty/serial: Add rx-tx-swap OF option to stm32-usart

STM32 F7/H7 usarts supports RX & TX pin swapping.
Add option to turn it on.
Tested on STM32MP157.

Acked-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: Martin Devera <devik@eaxlabs.cz>
Link: https://lore.kernel.org/r/20210328154306.22674-2-devik@eaxlabs.cz


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