arm64: dts: imx8mn: add clock for snvs rtc node

Initial commit adding imx8mn support:
6c3debcb

 ("arm64: dts: freescale: Add i.MX8MN dtsi support")
added the "clock-names" property for the snvs rtc node,
however it missed adding the clock.

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