dt-bindings: add binding for the Allwinner A64 DE2 bus

All the sub-blocks of Allwinner A64 DE2 needs the SRAM C on A64 SoC to
be claimed, otherwise the whole DE2 space is inaccessible.

Add a device tree binding of the DE2 part as a sub-bus.

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