kbuild: allow to run dt_binding_check and dtbs_check in a single command

Since commit 93512dad ("dt-bindings: Improve validation build error
handling"), 'make dtbs_check' does not validate the schema fully.

If you want to check everything, you need to run two commands separately.

  $ make ARCH=arm dt_binding_check
  $ make ARCH=arm dtbs_check

They are exclusive each other, so you cannot do like this:

  $ make ARCH=arm dt_binding_check dtbs_check

In this case, dt-doc-validate and dt-extract-example are skipped
because CHECK_DTBS is set.

Let's make it possible to run these two targets in a single command.
It will be useful for schema writers.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>