arm64: dts: imx8mm: add clock for SNVS RTC node

i.MX8MM has clock gate for SNVS module, add clock info to SNVS
RTC node for clock management.

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>