ppc4xx: Allow setting a single SPD EEPROM address for DDR2 DIMMs

On platforms where SPD EEPROM and another EEPROM have adjacent
I2C addresses SPD_EEPROM_ADDRESS should be defined as a single
element array, otherwise DDR2 setup code would fail with the
following error:

ERROR: Unknown DIMM detected in slot 1

However, fixing SPD_EEPROM_ADDRESS would result in another
error:

ERROR: DIMM's DDR1 and DDR2 type can not be mixed.

This happens because initdram() routine does not explicitly
initialize dimm_populated array. This patch fixes the problem.

Signed-off-by: Felix Radensky <felix@embedded-sol.com>
Signed-off-by: Stefan Roese <sr@denx.de>