clocksource/drivers/c-sky: fixup qemu fail to bootup sometimes.

Timer startup must after timer_irq_enable. For qemu, timer tick
irq hanppens but irq not enable, so it will cause qemu boot failed.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Tested-by: Liu Zhiwei <zhiwei_liu@c-sky.com>