dt-bindings: arm: Remove the CPU compatible documentation

Commit 1f8afea8fee0 ("dt-bindings: Add YAML description for Allwinner
boards") added an exhaustive list of the valid compatibles used for both
the SoCs and boards.

We can therefore remove the old documentation, that never got as well
updated as it should have.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>