Documentation: dtb: xgene: Add rxlos GPIO mapping

Signed-off-by: Quan Nguyen <[e-mail]>
Signed-off-by: Iyappan Subramanian <[e-mail]>
Tested-by: Fushen Chen <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/devicetree/bindings/net/apm-xgene-enet.txt

···
 	    Valid values are between 0 to 7, that maps to
 	    273, 589, 899, 1222, 1480, 1806, 2147, 2464 ps
 	    Default value is 2, which corresponds to 899 ps
+- rxlos-gpios: Input gpio from SFP+ module to indicate availability of
+	       incoming signal.
+
 
 Example:
 	menetclk: menetclk {