It wasn't obvious that this was a command to run based on 'make help',
so add it to the top-level help for devicetree builds. Also, add an
example to the documentation to show that db_binding_check can be run
with DT_SCHEMA_FILES= to only check one schema file instead of all of
them.

Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: <linux-kbuild@vger.kernel.org>
Cc: <devicetree@vger.kernel.org>
Cc: <linux-doc@vger.kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
[robh: fix-up due to .md to .rst conversion]
Signed-off-by: Rob Herring <robh@kernel.org>