rtc: snvs: Make SNVS clock always prepared

In IRQ handler, ONLY clock enable/disable is called due to
clock prepare can NOT be called in interrupt context, but
clock enable/disable will return failure if prepare count
is 0, to fix this issue, just make SNVS clock always prepared
there, the SNVS clock has no prepare function implemented,
so it won't impact anything.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/1590113996-31845-1-git-send-email-Anson.Huang@nxp.com