powerpc/8xxx: Enable DDR3 RDIMM support

Enabled registered DIMMs using data from SPD. RDIMMs have registers
which need to be configured before using. The register configuration
words are stored in SPD byte 60~116 (JEDEC standard No.21-C). Software
should read those RCWs and put into DDR controller before initialization.

Signed-off-by: York Sun <yorksun@freescale.com>