arm: dts: ls1028a-rdb: use Linux compatible string for RTC

During this board's sync with Linux device trees, it was observed that
it doesn't use the same compatible string for the RTC node as in U-Boot.
This change makes the RTC compatible strings match, for a smoother sync.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>