Fix DDR4_2400_8x8 DRAMCTRL configuration

Change-Id: I7af361e146909acc158590354ab22732d4b2f3d5
Signed-off-by: Wendy Elsasser <wendy.elsasser@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8101
Maintainer: Jason Lowe-Power <jason@lowepower.com>