dma/puv3_dma: Convert sysbus init function to realize function

dma/puv3_dma: Convert sysbus init function to realize function

Use DeviceClass rather than SysBusDeviceClass in
puv3_dma_class_init().

Cc: gxt@mprc.pku.edu.cn
Signed-off-by: Mao Zhongyi <maozhongyi@cmss.chinamobile.com>
Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181130093852.20739-7-maozhongyi@cmss.chinamobile.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>