timer/etraxfs_timer: Convert sysbus init function to realize function

timer/etraxfs_timer: Convert sysbus init function to realize function

Use DeviceClass rather than SysBusDeviceClass in
etraxfs_timer_class_init().

Cc: edgar.iglesias@gmail.com
Signed-off-by: Mao Zhongyi <maozhongyi@cmss.chinamobile.com>
Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 20181130093852.20739-17-maozhongyi@cmss.chinamobile.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>