devicetree: Expose dtbs_check and dt_binding_check some more

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>

Documentation/devicetree/writing-schema.rst

@@ -141,6 +141,7 @@ It is also possible to run checks with a single schema file by setting the
 
 ::
 
+    make dt_binding_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/trivial-devices.yaml
     make dtbs_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/trivial-devices.yaml
 
 

Makefile

@@ -1506,8 +1506,10 @@ help:
 	@echo  ''
 	@$(if $(dtstree), \
 		echo 'Devicetree:'; \
-		echo '* dtbs            - Build device tree blobs for enabled boards'; \
-		echo '  dtbs_install    - Install dtbs to $(INSTALL_DTBS_PATH)'; \
+		echo '* dtbs             - Build device tree blobs for enabled boards'; \
+		echo '  dtbs_install     - Install dtbs to $(INSTALL_DTBS_PATH)'; \
+		echo '  dt_binding_check - Validate device tree binding documents'; \
+		echo '  dtbs_check       - Validate device tree source files';\
 		echo '')
 
 	@echo 'Userspace tools targets:'