MPC8xxx: drop redundant boot messages

Current code would print RAM size information like this:

	DRAM:  DDR: 256 MiB (DDR1, 64-bit, CL=2, ECC off)

Turn a number of printf()s into debug() to get rid of the redundant
"DDR: " string like this:

	DRAM:  256 MiB (DDR1, 64-bit, CL=2, ECC off)

Signed-off-by: Wolfgang Denk <wd@denx.de>
Acked-by: York Sun <yorksun@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>