checkpatch.pl: Update DT vendor prefix check

In commit 8122de54 ("dt-bindings: Convert vendor prefixes to
json-schema"), vendor-prefixes.txt has been converted to a DT schema.
Update the checkpatch.pl DT check to extract vendor prefixes from the new
vendor-prefixes.yaml file.

Fixes: 8122de54

 ("dt-bindings: Convert vendor prefixes to json-schema")
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Rob Herring <robh@kernel.org>