powerpc/mpc85xx/p2020rdb-pca: Use L2 SRAM for SPL boot

This allows DDR configuration to be deferred to the final U-Boot image,
which is able to make use of SPD data.  The SPL itself cannot use SPD due
to code size constraints.  It previously used fixed register values for
DDR configuration, and those values did not work on the p2020rdb-pca
board I tested with.  It's possible that different revisions of the board
require different settings.  Using SPD eliminates that problem.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: Andy Fleming <afleming@freescale.com>