clk: imx8mn: add SNVS clock to clock tree

i.mx8mn has support for clock gating the snvs module.
Add it into clock tree so that rtc-snvs driver could use it.

Note this will also be required in the snvs_pwrkey driver,
once support for clock management will be added.

Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>