dt-bindings: Output yamllint warnings to stderr

yamllint warnings go to stdout which means on a quiet build no warnings
are output. Fix this and redirect the yamllint output to stderr.
Signed-off-by: NRob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210820000047.1667819-1-robh@kernel.org

Documentation/devicetree/bindings/Makefile

@@ -28,7 +28,7 @@ find_cmd = find $(srctree)/$(src) \( -name '*.yaml' ! \
 
 quiet_cmd_yamllint = LINT    $(src)
       cmd_yamllint = ($(find_cmd) | \
-                     xargs $(DT_SCHEMA_LINT) -f parsable -c $(srctree)/$(src)/.yamllint) || true
+                     xargs $(DT_SCHEMA_LINT) -f parsable -c $(srctree)/$(src)/.yamllint >&2) || true
 
 quiet_cmd_chk_bindings = CHKDT   $@
       cmd_chk_bindings = ($(find_cmd) | \