Documentation/bindings: net: the sfp i2c-bus property is now mandatory

The i2c-bus property for sfp modules was made mandatory. Update the
documentation to keep it in sync with the driver's behaviour.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/sff,sfp.txt

@@ -7,11 +7,11 @@ Required properties:
   "sff,sfp" for SFP modules
   "sff,sff" for soldered down SFF modules
 
-Optional Properties:
-
 - i2c-bus : phandle of an I2C bus controller for the SFP two wire serial
   interface
 
+Optional Properties:
+
 - mod-def0-gpios : GPIO phandle and a specifier of the MOD-DEF0 (AKA Mod_ABS)
   module presence input gpio signal, active (module absent) high. Must
   not be present for SFF modules