dt-bindings: add compatible string for Allwinner R40 CCU

Allwinner R40 has a clock controlling unit like the ones on other
Allwinner SoCs after sun6i, and can also use a CCU-based driver.

Add a compatible string for it.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>