learning_gem5,configs: Update ruby_test

Use SimpleMemory instead of DDR3 so we can use the timing results in
tests. By using SimpleMemory, even if the DRAM timing changes the timing
of this test won't change. I expect the timing of SimpleMemory to never
change.

Change-Id: I4c75981d7b8bfc4dcca59e628e89f5a6ea4c0e36
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17871
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>

configs/learning_gem5/part3/ruby_test.py

@@ -61,8 +61,8 @@ system.tester = RubyTester(checks_to_complete = 100,
                            wakeup_frequency = 10,
                            num_cpus = 2)
 
-# Create a DDR3 memory controller and connect it to the membus
-system.mem_ctrl = DDR3_1600_8x8()
+# Create a simple memory controller and connect it to the membus
+system.mem_ctrl = SimpleMemory(latency="50ns", bandwidth="0GB/s")
 system.mem_ctrl.range = system.mem_ranges[0]
 
 # Create the Ruby System