dt-bindings: serial: stm32: add RS485 optional properties

Add options for enabling RS485 hardware control and configuring
Driver Enable signal:
- rs485-rts-delay
- rs485-rx-during-tx
- rs485-rts-active-low
- linux,rs485-enabled-at-boot-time

Signed-off-by: Bich Hemon <bich.hemon@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>