mpc512x: fix fixed_sdram() init code.

Commit 054197ba and later fixes used an array to initialize some of
the MDDRC parameters; however, the use of an array turned out to be a
bad idea as it was not possible to correlate structure entries to
array indices in readable and reliable way. Now we use a struct
instead, which makes this self-explanatory.

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