ARM: dts: imx7s: add snvs clock to pwrkey

On i.MX7, the SNVS requires a clock. This is similar to the clock
bound to the SNVS RTC node, but if the SNVS RTC driver isn't enabled,
then SNVS doesn't work, and as such the pwrkey driver doesn't
work (i.e. hangs the kernel, as the clock isn't enabled).

Also see commit ec2a844e
("ARM: dts: imx7s: add snvs rtc clock")
for a similar fix.

Signed-off-by: André Draszik <git@andred.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>