dt-bindings: kbuild: Add separate target/dependency for processed-schema.json

Running dtbs_check and dt_compatible_check targets really only depend
on processed-schema.json, but the dependency is 'dt_binding_check'. That
was sort worked around with the CHECK_DT_BINDING variable in order to
skip some of the work that 'dt_binding_check' does. It still runs the
full checks of the schemas which is not necessary and adds 10s of
seconds to the build time. That's significant when checking only a few
DTBs and with recent changes that have improved the validation time by
6-7x.

Add a new target, dt_binding_schema, which just builds
processed-schema.json and can be used as the dependency for other
targets. The scripts_dtc dependency isn't needed either as the examples
aren't built for it.

Signed-off-by: Rob Herring <robh@kernel.org>
Tested-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>