In commit 796af3473b82 ("dts: update to v5.2-rc1")
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.

Based on this linux kernel commit 852d095d16a6
("checkpatch.pl: Update DT vendor prefix check").

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>