Updates to common PPC4xx onboard (DDR)SDRAM init code (405 and 440)

405 SDRAM: - The SDRAM parameters can now be defined in the board
             config file and the 405 SDRAM controller values will
             be calculated upon bootup (see PPChameleonEVB).
             When those settings are not defined in the board
             config file, the register setup will be as it is now,
             so this implementation should not break any current
             design using this code.

             Thanks to Andrea Marson from DAVE for this patch.

440 DDR:   - Added function sdram_tr1_set to auto calculate the
             TR1 value for the DDR.
           - Added ECC support (see p3p440).

Patch by Stefan Roese, 17 Mar 2006