dt-bindings: Add running yamllint to dt_binding_check

Add a yamllint config file and support for running yamllint on DT
binding schema files. This runs on the whole tree as yamllint is Python
and suffers from Python's slow startup times.

Users can run on individual files doing:

yamllint -c Documentation/devicetree/bindings/.yamllint <binding file>

Link: https://lore.kernel.org/r/20201009170557.168785-1-robh@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/.yamllint

+extends: relaxed
+
+rules:
+  line-length:
+    # 80 chars should be enough, but don't fail if a line is longer
+    max: 110
+    allow-non-breakable-words: true
+    level: warning
+  braces:
+    min-spaces-inside: 0
+    max-spaces-inside: 1
+    min-spaces-inside-empty: 0
+    max-spaces-inside-empty: 0
+  brackets:
+    min-spaces-inside: 0
+    max-spaces-inside: 1
+    min-spaces-inside-empty: 0
+    max-spaces-inside-empty: 0
+  colons: {max-spaces-before: 0, max-spaces-after: 1}
+  commas: {min-spaces-after: 1, max-spaces-after: 1}
+  comments:
+    require-starting-space: false
+    min-spaces-from-content: 1
+  comments-indentation: disable
+  document-start:
+    present: true
+  empty-lines:
+    max: 3
+    max-end: 1
+  empty-values:
+    forbid-in-block-mappings: true
+    forbid-in-flow-mappings: true
+  hyphens:
+    max-spaces-after: 1
+  indentation:
+    spaces: 2
+    indent-sequences: true
+    check-multi-line-strings: false
+  trailing-spaces: false

Documentation/devicetree/bindings/Makefile

@@ -3,6 +3,8 @@ DT_DOC_CHECKER ?= dt-doc-validate
 DT_EXTRACT_EX ?= dt-extract-example
 DT_MK_SCHEMA ?= dt-mk-schema
 
+DT_SCHEMA_LINT = $(shell which yamllint)
+
 DT_SCHEMA_MIN_VERSION = 2020.8.1
 
 PHONY += check_dtschema_version
@@ -24,6 +26,10 @@ find_cmd = find $(srctree)/$(src) \( -name '*.yaml' ! \
 		-name 'processed-schema*' ! \
 		-name '*.example.dt.yaml' \)
 
+quiet_cmd_yamllint = LINT    $(src)
+      cmd_yamllint = $(find_cmd) | \
+                     xargs $(DT_SCHEMA_LINT) -f parsable -c $(srctree)/$(src)/.yamllint
+
 quiet_cmd_chk_bindings = CHKDT   $@
       cmd_chk_bindings = $(find_cmd) | \
                          xargs -n200 -P$$(nproc) $(DT_DOC_CHECKER) -u $(srctree)/$(src)
@@ -37,6 +43,7 @@ quiet_cmd_mk_schema = SCHEMA  $@
 		      rm -f $$f
 
 define rule_chkdt
+	$(if $(DT_SCHEMA_LINT),$(call cmd,yamllint),)
 	$(call cmd,chk_bindings)
 	$(call cmd,mk_schema)
 endef
@@ -48,7 +55,7 @@ override DTC_FLAGS := \
 	-Wno-graph_child_address \
 	-Wno-interrupt_provider
 
-$(obj)/processed-schema-examples.json: $(DT_DOCS) check_dtschema_version FORCE
+$(obj)/processed-schema-examples.json: $(DT_DOCS) $(src)/.yamllint check_dtschema_version FORCE
 	$(call if_changed_rule,chkdt)
 
 ifeq ($(DT_SCHEMA_FILES),)