rtc: pcf2127: remove U-Boot specific compatible string

Now that all in-tree boards have been converted to the compatible
strings from Linux, delete the support for the ad-hoc "pcf2127-rtc" one.

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>