arm64: dts: imx8mm-kontron: Add RTC aliases

Add aliases for the RTCs on the board and on the SoC. This ensures that
the primary RTC is always the one on the board that has a buffered supply
and maximum accuracy.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>