86xx: Allow for fewer DDR slots per memory controller.

As a direct correlation exists between DDR DIMM slots
and SPD EEPROM addresses used to configure them, use
the individually defined SPD_EEPROM_ADDRESS* values to
determine if a DDR DIMM slot should have its SPD
configuration read or not.

Effectively, this now allows for 1 or 2 DIMM slots
per memory controller.

Signed-off-by: Jon Loeliger <jdl@freescale.com>