devicetree: add vendor prefix for National Instruments

Signed-off-by: NNathan Sullivan <nathan.sullivan@ni.com>
Signed-off-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -191,6 +191,7 @@ netgear	NETGEAR
 netlogic	Broadcom Corporation (formerly NetLogic Microsystems)
 netxeon		Shenzhen Netxeon Technology CO., LTD
 newhaven	Newhaven Display International
+ni	National Instruments
 nintendo	Nintendo
 nokia	Nokia
 nuvoton	Nuvoton Technology Corporation