Add a make target, dt_compatible_check, to extract compatible strings
from kernel sources and check if they are documented by a schema.
At least version v2022.08 of dtschema with dt-check-compatible is
required.

This check can also be run manually on specific files or directories:

scripts/dtc/dt-extract-compatibles drivers/clk/ | \
  xargs dt-check-compatible -v -s Documentation/devicetree/bindings/processed-schema.json

Currently, there are about 3800 undocumented compatible strings. Most of
these are cases where the binding is not yet converted (given there
are 1900 .txt binding files remaining).

Link: https://lore.kernel.org/all/20220916012510.2718170-1-robh@kernel.org/Signed-off-by: Rob Herring <robh@kernel.org>