mpc512x: Streamlined fixed_sdram() init sequence.

Signed-off-by: Martha M Stan <mmarx@silicontkx.com>

Minor cleanup:

Re-ordered default_mddrc_config[] to have matching indices.

This allows to use the same index "N" for source and target fields;
before, we had code like this

	out_be32(&im->mddrc.ddr_time_config2, mddrc_config[3]);

which always looked like a copy & paste error because 2 != 3.

Also, use NULL when meaning a null pointer.

Signed-off-by: Wolfgang Denk <wd@denx.de>