dt-bindings: sram: sunxi-sram: Clean up the compatible lists

Use enumerations where appropriate to combine "const" choices and
deduplicate fallback compatible strings.

Suggested-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Link: https://lore.kernel.org/r/20220815041248.53268-2-samuel@sholland.org


Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>