ARM: mvebu: Enable ISL12057 RTC chip in ReadyNAS 2120 .dts file

Now that support for Intersil ISL12057 RTC chip is available
upstream, let's enable it in NETGEAR ReadyNAS 2120 .dts file
so that the device stop believing it's the 70's.

Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>