Makefile: Add support for DT bindings schema checks

Makefile: Add support for DT bindings schema checks

This adds the build infrastructure for checking DT binding schema
documents and validating dtb files using the binding schema. Here we use
devicetree-rebasing subtree to provide the DT bindings. Along with that
adapt dts/upstream/Bindings/Makefile to align with old U-Boot Kbuild
infrastructure.

Dependency:
-----------

The DT schema project must be installed in order to validate the DT schema
binding documents and validate DTS files using the DT schema. The DT schema
project can be installed with pip::

    pip3 install dtschema

Note that 'dtschema' installation requires 'swig' and Python development
files installed first. On Debian/Ubuntu systems::

    apt install swig python3-dev

Testing:
--------

Build dts files and check using DT binding schema:
$ make dtbs_check

Optionally, DT_SCHEMA_FILES can be passed in with a schema file(s) to
use for validation. This makes it easier to find and fix errors
generated by a specific schema.

Note, at this point dtbs_check is an optional build target as there are
many warnings generated due to custom DT properties used by many
platforms in u-boot. It is expected with these checks that compliance
with DT bindings to take place. Once that's done it can be added to CI
builds to remain compliant with DT bindings.

Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>