powerpc/mpc83xx: Migrate from spd_sdram to unified DDR driver

Unified DDR driver is maintained for better performance, robustness and bug
fixes. Upgrading to use unified DDR driver for MPC83xx takes advantage of
overall improvement. It requires changes for board files to customize
platform-dependent parameters.

To utilize the unified DDR driver, a board needs to define CONFIG_FSL_DDRx
in the header file. No more boards will be accepted without such definition.

Note: the workaround for erratum DDR6 for the very old MPC834x Rev 1.0/1.1
and MPC8360 Rev 1.1/1.2 parts is not migrated to unified driver.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>