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>

Documentation/devicetree/bindings/clock/sunxi-ccu.txt

@@ -12,6 +12,7 @@ Required properties :
 		- "allwinner,sun8i-a83t-r-ccu"
 		- "allwinner,sun8i-h3-ccu"
 		- "allwinner,sun8i-h3-r-ccu"
++		- "allwinner,sun8i-r40-ccu"
 		- "allwinner,sun8i-v3s-ccu"
 		- "allwinner,sun9i-a80-ccu"
 		- "allwinner,sun50i-a64-ccu"