dt-bindings: sram: Convert Allwinner A10 system controller to a schema

The older Allwinner SoCs have a system controller that is supported in
Linux, with a matching Device Tree binding.

Now that we have the DT validation in place, let's convert the device tree
bindings for that controller over to a YAML schemas.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
[robh: fixup missing 'properties']
Signed-off-by: Rob Herring <robh@kernel.org>