checkpatch: add DT compatible string documentation checks

This adds a simple check that any compatible strings in DeviceTree dts
files are present in Documentation/devicetree/bindings.  Vendor prefixes
are also checked for existing in vendor-prefixes.txt These should be
temporary checks until we have more sophisticated binding schema
checking.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Joe Perches <joe@perches.com>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Git-commit: bff5da4335256513497cc8c79f9a9d1665e09864
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>