powerpc/mpc85xx: Revise workaround for DDR-A003

Existing workaround only handles one RDIMM on reference design. In case
of two RDIMMs being used, the workaround requires two separate writes to
DDR_SDRAM_MD_CNTL register.

This patch also restores two debug registers changed by the workaround.

Signed-off-by: York Sun <yorksun@freescale.com>
CC: Ben Collins <ben.c@servergy.com>
CC: James Yang <James.Yang@freescale.com>