dt-bindings: add binding for the Allwinner DE2 CCU

Allwinner "Display Engine 2.0" contains some clock controls in it.

In order to add them as clock drivers, we need a device tree binding.
Add the binding here.

Also add the device tree binding headers.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>