dt-bindings: slave-device: add current-speed property

This adds a new DT property to define the current baud rate of the
slave device.
Signed-off-by: NStefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/serial/slave-device.txt

@@ -21,6 +21,15 @@ Optional Properties:
 		  can support. For example, a particular board has some signal
 		  quality issue or the host processor can't support higher
 		  baud rates.
+- current-speed	: The current baud rate the device operates at. This should
+		  only be present in case a driver has no chance to know
+		  the baud rate of the slave device.
+		  Examples:
+		    * device supports auto-baud
+		    * the rate is setup by a bootloader and there is no
+		      way to reset the device
+		    * device baud rate is configured by its firmware but
+		      there is no way to request the actual settings
 
 Example: