powerpc/8xxx: Add support for interactive DDR programming interface

Interactive DDR debugging provides a user interface to view and modify SPD,
DIMM parameters, board options and DDR controller registers before DDR is
initialized. With this feature, developers can fine-tune DDR for board
bringup and other debugging without frequently having to reprogram the flash.

To enable this feature, define CONFIG_FSL_DDR_INTERACTIVE in board header
file and set an environment variable to activate it. Syntax:

setenv ddr_interactive on

After reset, U-boot prompts before initializing DDR controllers
FSL DDR>

The available commands are
print      print SPD and intermediate computed data
reset      reboot machine
recompute  reload SPD and options to default and recompute regs
edit       modify spd, parameter, or option
compute    recompute registers from current next_step to end
next_step  shows current next_step
help       this message
go         program the memory controller and continue with u-boot

The first command should be "compute", which reads data from DIMM SPDs and
board options, performs the calculation then stops before setting DDR
controller. A user can use "print" and "edit" commands to view and modify
anything. "Go" picks up from current step with any modification and
compltes the calculation then enables the DDR controller to continue u-boot.
"Recompute" does it over from fresh reading.

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