sparc: time: Replace update_persistent_clock() with CONFIG_RTC_SYSTOHC

On Sparc systems, update_persistent_clock() uses RTC drivers to do
the job, it makes more sense to hand it over to CONFIG_RTC_SYSTOHC.

In the long run, all the update_persistent_clock() should migrate to
proper class RTC drivers if any and use CONFIG_RTC_SYSTOHC instead.

Signed-off-by: Xunlei Pang <pang.xunlei@linaro.org>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>