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.be


Signed-off-by: Rob Herring <robh@kernel.org>