Add AD Holdings Plc. to vendor-prefixes.

AD Holdings design and manufacture IP Camera technology in the UK.

http://www.ad-group.co.uk/

Signed-off-by: Ian Molton <imolton@ad-holdings.co.uk>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -7,6 +7,7 @@ abilis	Abilis Systems
 active-semi	Active-Semi International Inc
 ad	Avionic Design GmbH
 adapteva	Adapteva, Inc.
+adh	AD Holdings Plc.
 adi	Analog Devices, Inc.
 aeroflexgaisler	Aeroflex Gaisler AB
 allwinner	Allwinner Technology Co., Ltd.