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>

src/mem/DRAMCtrl.py

@@ -642,6 +642,9 @@ class DDR4_2400_8x8(DDR4_2400_16x4):
     # Each device has a page (row buffer) size of 1 Kbyte (1K columns x8)
     device_rowbuffer_size = '1kB'
 
+    # 8x8 configuration, so 8 devices
+    devices_per_rank = 8
+
     # RRD_L (same bank group) for 1K page is MAX(4 CK, 4.9ns)
     tRRD_L = '4.9ns';