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>