dt-bindings: serial: stm32: support new rs485 delays in nanoseconds

Add rs485-rts-delay-ns (in nanoseconds) property.
This extends the range of delay values for user to microseconds and
nanoseconds.

With fast data rates (1Mbaut or more), 1ms become quite long. Users need
to be able to set shorter delays than 1 ms in order not to slow down the
channel unnecessarily. This enables faster communication channels without
changing the baudrate.

Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Change-Id: I9a1b8759440b9643760cfdc3d843a6b4a830a9f8

Author: Erwan Le Ray

Documentation/devicetree/bindings/serial/st,stm32-uart.yaml

@@ -59,6 +59,7 @@ properties:
   wakeup-source: true
 
   rs485-rts-delay: true
+  rs485-rts-delay-ns: true
   rs485-rts-active-low: true
   linux,rs485-enabled-at-boot-time: true
   rs485-rx-during-tx: true