pci-bridge/dec: Convert sysbus init function to realize function

pci-bridge/dec: Convert sysbus init function to realize function

Use DeviceClass rather than SysBusDeviceClass in
pci_dec_21154_device_class_init().

Cc: david@gibson.dropbear.id.au
Cc: mst@redhat.com
Cc: marcel.apfelbaum@gmail.com
Cc: qemu-ppc@nongnu.org
Signed-off-by: Mao Zhongyi <maozhongyi@cmss.chinamobile.com>
Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Message-id: 20181130093852.20739-16-maozhongyi@cmss.chinamobile.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>