dt-bindings: Add vendor prefix for AsusTek Computer Inc.

"AsusTek Computer Inc. is a Taiwanese multinational computer
and phone hardware and electronics company headquartered
in Beitou District, Taipei, Taiwan." - Wikipedia.org

Website: https://www.asus.com

The prefix is already in use by at least 5 ARM boards
Signed-off-by: Luis Araneda <luaraneda@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -41,6 +41,7 @@ arrow	Arrow Electronics
 artesyn	Artesyn Embedded Technologies Inc.
 asahi-kasei	Asahi Kasei Corp.
 aspeed	ASPEED Technology Inc.
+asus	AsusTek Computer Inc.
 atlas	Atlas Scientific LLC
 atmel	Atmel Corporation
 auo	AU Optronics Corporation