mpc83xx: implement board_add_ram_info

add board_add_ram_info, to make memory diagnostic output more
consistent. u-boot banner output now looks like:

DRAM:  256 MB (DDR1, 64-bit, ECC on)

and for boards with SDRAM on the local bus, a line such as this is
added:

SDRAM: 64 MB (local bus)

also replaced some magic numbers with their equivalent define names.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>