drivers/rtc/rtc-snvs: add clock support

Add clock enable and disable support for the SNVS peripheral, which is
required for using the RTC within the SNVS block.

The clock is not strictly enforced, as this would break the i.MX devices.
The clocking for the i.MX devices seems to be enabled elsewhere and
enabling RTC SNVS for Vybrid results in a crash.  This patch adds the
clock support but also makes it optional so Vybrid platform can use the
clock if defined while making sure not to break i.MX.

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Stefan Agner <stefan@agner.ch>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>