dt-bindings: net: sms911x: Add missing optional VDD regulators

The lan911x family of devices require supplying from 3.3 V power
supplies (connected to VDD_IO, VDD_A and VREG_3.3 pins).  The existing
driver however obtains only VDD_IO and VDD_A regulators in an optional
way so document this in bindings.
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/smsc911x.txt

@@ -27,6 +27,7 @@ Optional properties:
   of the device. On many systems this is wired high so the device goes
   out of reset at power-on, but if it is under program control, this
   optional GPIO can wake up in response to it.
+- vdd33a-supply, vddvario-supply : 3.3V analog and IO logic power supplies
 
 Examples: