MLK-11360-05 ARM: dtsi: imx7d: add snvs clock for rtc, power key, and caam nodes

Since snvs-rtc, snvs-pwrkey, snvs-caam all have snvs HP register access
that need snvs clock root enable. Once snvs clock root is defined in i.MX7d
clock driver, it requires device tree to define the clocks property for
snvs related modules like rtc, power key, caam modules.

Signed-off-by: Fugang Duan <B38611@freescale.com>