mx25pdk: Enable imxdi RTC

The mx25pdk board supports the i.MX25 DryIce RTC (imxdi), so enable it. This
allows to compile-test the imxdi driver in the mainline tree.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Acked-by: Fabio Estevam <fabio.estevam@freescale.com>