dt-bindings: Convert vendor prefixes to json-schema

Convert the vendor prefix registry to a schema. This will enable checking
that new vendor prefixes are added (in addition to the less than perfect
checkpatch.pl check) and will also check against adding other prefixes
which are not vendors.

Converted vendor-prefixes.txt using the following sed script:

sed -e 's/\([a-zA-Z0-9\-]*\)[[:space:]]*\([a-zA-Z0-9].*\)/  "^\1,\.\*\":\n    description: \2/'
Signed-off-by: Rob Herring <robh@kernel.org>