powerpc/8xxx: Refactor fsl_ddr_get_spd into common code from board

Move fsl_ddr_get_spd into common mpc8xxx/ddr/main.c as most boards
pretty much do the same thing.  The only variations are in how many
controllers or DIMMs per controller exist.  To make this work we
standardize on the names of the SPD_EEPROM_ADDRESS defines based on the
use case of the board.

We allow boards to override get_spd to either do board specific fixups
to the SPD data or deal with any unique behavior of how the SPD eeproms
are wired up.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>