dt-bindings: add binding for Allwinner A64 SRAM controller and SRAM C

The display engine on Allwinner A64 wants to claim the SRAM C section.

Add a SRAM controller compatible for A64, and a SRAM section compatible
for its SRAM C.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>