dt-bindings: add device tree binding for Allwinner H6 main CCU

The Allwinner H6 main CCU uses the internal oscillator of the SoC, which
is different with old SoCs' main CCU.

Add device tree binding for the Allwinner H6 main CCU.
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>

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

@@ -20,6 +20,7 @@ Required properties :
 		- "allwinner,sun50i-a64-ccu"
 		- "allwinner,sun50i-a64-r-ccu"
 		- "allwinner,sun50i-h5-ccu"
+		- "allwinner,sun50i-h6-ccu"
 		- "nextthing,gr8-ccu"
 
 - reg: Must contain the registers base address and length
@@ -31,6 +32,9 @@ Required properties :
 - #clock-cells : must contain 1
 - #reset-cells : must contain 1
 
+For the main CCU on H6, one more clock is needed:
+- "iosc": the SoC's internal frequency oscillator
+
 For the PRCM CCUs on A83T/H3/A64, two more clocks are needed:
 - "pll-periph": the SoC's peripheral PLL from the main CCU
 - "iosc": the SoC's internal frequency oscillator