mem: Update DRAM configuration names

Names of DRAM configurations were updated to reflect both
the channel and device data width.

Previous naming format was:
	<DEVICE_TYPE>_<DATA_RATE>_<CHANNEL_WIDTH>

The following nomenclature is now used:
	<DEVICE_TYPE>_<DATA_RATE>_<n>x<w>
where n = The number of devices per rank on the channel
      x = Device width

Total channel width can be calculated by n*w

Example:
A 64-bit DDR4, 2400 channel consisting of 4-bit devices:
	n = 16
	w = 4
The resulting configuration name is:
	DDR4_2400_16x4

Updated scripts to match new naming convention.

Added unique configurations for DDR4 for:
1) 16x4
2) 8x8
3) 4x16

Change-Id: Ibd7f763b7248835c624309143cb9fc29d56a69d1
Reviewed-by: Radhika Jagtap <radhika.jagtap@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>

configs/learning_gem5/part1/simple.py

@@ -75,7 +75,7 @@ if m5.defines.buildEnv['TARGET_ISA'] == "x86":
     system.cpu.interrupts[0].int_slave = system.membus.master
 
 # Create a DDR3 memory controller and connect it to the membus
-system.mem_ctrl = DDR3_1600_x64()
+system.mem_ctrl = DDR3_1600_8x8()
 system.mem_ctrl.range = system.mem_ranges[0]
 system.mem_ctrl.port = system.membus.master