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>