rtc: snvs: Add necessary clock operations for RTC APIs

There could be still RTC registers access after RTC suspend
with clock disabled, need to add clock operations for each
RTC API to make sure accessing RTC registers is successfully.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/1590113996-31845-2-git-send-email-Anson.Huang@nxp.com