dt-bindings: Add Ubiquiti Networks vendor prefix

Use the stock ticker: UBNT as the vendor prefix for Ubiquiti Networks.
Acked-by: NJon Mason <jon.mason@broadcom.com>
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -351,6 +351,7 @@ truly	Truly Semiconductors Limited
 tsd	Theobroma Systems Design und Consulting GmbH
 tyan	Tyan Computer Corporation
 ucrobotics	uCRobotics
+ubnt	Ubiquiti Networks
 udoo	Udoo
 uniwest	United Western Technologies Corp (UniWest)
 upisemi	uPI Semiconductor Corp.