am335x_evm / gumstix pepper: Correct DDR settings

As noted by clang, we have been shifting certain values out of 32bit
range when setting some DDR registers.  Upon further inspection these
had been touching reserved fields (and having no impact).  These came in
from historical bring-up code and can be discarded.  Similarly, we had
been declaring some fields as 0 when they will be initialized that way.
Tested on Beaglebone White.

Reported-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Cc: Ash Charles <ash@gumstix.com>
Signed-off-by: Tom Rini <trini@ti.com>
Tested-By: Ash Charles <ashcharles@gmail.com>