dt-bindings: Don't error out on yamllint and dt-doc-validate errors

A broken schema file now causes make to exit and 'make -k' no longer works
now that dt-doc-validate is called from a single make rule.

As yamllint is optional, we shouldn't stop on yamllint errors either. Also,
it seems some old versions of yamllint don't work.
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/Makefile

@@ -27,12 +27,12 @@ find_cmd = find $(srctree)/$(src) \( -name '*.yaml' ! \
 		-name '*.example.dt.yaml' \)
 
 quiet_cmd_yamllint = LINT    $(src)
-      cmd_yamllint = $(find_cmd) | \
-                     xargs $(DT_SCHEMA_LINT) -f parsable -c $(srctree)/$(src)/.yamllint
+      cmd_yamllint = ($(find_cmd) | \
+                     xargs $(DT_SCHEMA_LINT) -f parsable -c $(srctree)/$(src)/.yamllint) || true
 
 quiet_cmd_chk_bindings = CHKDT   $@
-      cmd_chk_bindings = $(find_cmd) | \
-                         xargs -n200 -P$$(nproc) $(DT_DOC_CHECKER) -u $(srctree)/$(src)
+      cmd_chk_bindings = ($(find_cmd) | \
+                         xargs -n200 -P$$(nproc) $(DT_DOC_CHECKER) -u $(srctree)/$(src)) || true
 
 quiet_cmd_mk_schema = SCHEMA  $@
       cmd_mk_schema = f=$$(mktemp) ; \