clocksource/drivers/sp804: Use clk_prepare_enable and clk_disable_unprepare

Directly use clk_prepare_enable and clk_disable_unprepare.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201029123317.90286-3-wangkefeng.wang@huawei.com