dt-bindings: Add vendor prefix for Adaptrum, Inc.

Adaptrum is a manufacturer of TV White Space (TVWS) wireless
technology. Using dynamic spectrum access to deliver affordable
internet connectivity over non-line-of-sight (NLOS) fixed wireless,
Adaptrum is helping close the digital divide worldwide.

Signed-off-by: Alexandru Gagniuc <alex.g@adaptrum.com>
Signed-off-by: Rob Herring <robh@kernel.org>