arm64: dts: imx8mm: Model PMIC to SNVS RTC clock path on Data Modul i.MX8M Mini eDM SBC

The PMIC is the 32 kHz clock source for the RTC_XTALI input of the SoC
on this system. The RTC_XTALI input is used to supply 32 kHz clock to
the SVNS RTC per "i.MX 8M Mini Applications Processor Reference Manual,
Rev. 3, 11/2020" page 759 "The 32KHz XTAL module uses a different IP and
it is used as the clock source for the RTC, located in the SNVS." The
PMIC has its own dedicated 32 kHz XTAL on input.

Model the connection in DT.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>