configs: Fix NULL dram-lowp regressions

configs: Fix NULL dram-lowp regressions

The problem arises since there are some scripts (like NULL dram
regressions) which are making use of MemConfig.py without using
Opions.py so they won't have the new enable-dram-powerdown option

Change-Id: Id9769cce2e8a25b57da76f07eeebd279a6e00440
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19268
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>