ppc: qemu: Enable RTC support via I2C

The QEMU ppce500 target integrates a Freescale I2C controller and
has a Pericom pt7c4338 RTC connected to it. Enable corresponding
DM drivers so that 'date' command is actually useful.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>