rtc: pcf2127: sync with Linux compatible strings

Allow this driver to be used by boards which inherit their device trees
from Linux. Compatibility is temporarily retained with the old
compatible string which is U-Boot specific, and will be removed after a
few changes.

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