arm64: dts: imx8mm-beacon-som: Configure RTC aliases

On the i.MX8MM Beacon SOM, there is an RTC chip which is fed power
from the baseboard during power off.  The SNVS RTC integrated into
the SoC is not fed power.  Depending on the order the modules are
loaded, this can be a problem if the external RTC isn't rtc0.

Make the alias for rtc0 point to the external RTC all the time and
rtc1 point to the SVNS in order to correctly hold date/time over
a power-cycle.

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>