hw: arm: allwinner-sramc: Set class_size

AwSRAMCClass is larger than SysBusDeviceClass so the class size must be
advertised accordingly.

Fixes: 05def917 ("hw: arm: allwinner-sramc: Add SRAM Controller support for R40")
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230628110905.38125-1-akihiko.odaki@daynix.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>