dt-bindings: Parallelize yamllint

Use xargs sharding like "chk_bindings" does, to parallelize the
execution of yamllint.

This reduces the yamllint execution time from ca. 21 to 5 seconds on
i7-8700K.
Suggested-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/c68bdbdf3112c4658e0748c8fc51959e69fbae2e.1634551582.git.geert+renesas@glider.beSigned-off-by: Rob Herring <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 >&2) || true
+                     xargs -n200 -P$$(nproc) $(DT_SCHEMA_LINT) -f parsable -c $(srctree)/$(src)/.yamllint >&2) || true
 
 quiet_cmd_chk_bindings = CHKDT   $@
       cmd_chk_bindings = ($(find_cmd) | \