dt-bindings: bus: add H6 DE3 bus binding

The Allwinner H6 DE3 bus is similar to the A64 DE2 one.

Add its compatible string with the A64 string as fallback to the
binding.

Some description of the binding is modified to make it more generic.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
[Fixed compatible name]
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181104182705.18047-2-jernej.skrabec@siol.net