clocksource/drivers/sp804: Cleanup clk_get_sys()

Move the clk_get_sys() part into sp804_get_clock_rate(), cleanup the same
code.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200918132237.3552-2-thunder.leizhen@huawei.com