am33xx/ddr.c: Fix regression on DDR2 platforms

Back in fc46bae2 a "clean up" was introduced that intended to reconcile
some of the AM335x codepaths based on how AM43xx operates.
Unfortunately this introduced a regression on the DDR2 platforms.  This
was un-noticed on DDR3 (everything except for Beaglebone White) as we
had already populated sdram_config correctly in sequence.  This change
brings us back to the older behavior and is fine on all platforms.

Tested on Beaglebone White, Beaglebone Black and AM335x GP EVM

Reported-by: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>