powerpc/5200: Add digsy-mtc support to mpc5200_defconfig

The following options are enabled to support the digsy-mtc.
 - LXT phy
 - AT24 eeprom
 - RTC (DS1337)
 - MTD partitioning based on OF description

Signed-off-by: Grzegorz Bernacki <gjb@semihalf.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>