arm: dts: lx2160a-rdb: use Linux compatible string for RTC

During the LS1028A-RDB sync with Linux device trees, it was observed
that the same RTC is present on the two boards, and the wrong compatible
string is used in both places. This change updates the RTC from the
LX2160A-RDB to use the compatible string that was established in Linux.

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