dt-bindings: List devicetree binding for the CCU of Allwinner A20

Allwinner A20 is now driven by sunxi-ng CCU driver.

Add devicetree binding for it.

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Priit Laes <plaes@plaes.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>