clk: imx8mm: add SNVS clock to clock tree

clk: imx8mm: add SNVS clock to clock tree

i.MX8MM has clock gate for SNVS module, add it into clock tree
for SNVS RTC driver to manage.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>