powerpc/8xxx: Fix merge issue with P2020DS DDR2 build config

When P2020DS DDR2 was merged it was merged incorrectly and propogated to
boards.cfg.  Fix this by moving DDR2 config to be associated with
P2020DS and not P1_P2_RDB.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>