DMIPs in multi core systems
Um, strictly speaking: 

DMIPs is a "synthetic" ranking, based on how long it takes to execute a specific program, the Dhrystone testbench.

You just run that on your system. If your system can execute that fast, it gets high DMIPS. Since that benchmark itself doesn't take any multiprocessing into account, it's just a straightforward single-threaded C program, you'll probably not get any benefit from having a multicore system. (unless you have a special compiler that automatically distributes loads to different CPU cores – but then you're more benchmarking that auto-SMP-algorithm than your machine)

The different cluster modes you mention might, or might not, have an influence on Dhrystone rating. However, be very aware that if you run a synthetic benchmark while doing anything else on your CPUs, then you're doing something irresponsible – suddenly, the load that your other things put on the shared resources (mainly: memory buses, but also, the CPU core that your benchmark runs on, potentially, through thermal and barrier restrictions) will have an untraceable amount of influence on your Dhrystone performance. 

Generally, Dhrystone isn't used as "serious" benchmark suite, anymore, because it was written in 1984, and was a benchmark suite that tried to "emulate" typical workloads at the time. CPUs, computer architectures, and workloads have changed significantly in the 36 years from then that it's not anymore a representation of any interesting workload performance. You can even see that  in your case – because such systems effectively didn't exist in 1984, multi-core multi-processor die clusters simply can't be sensibly measured with it. However, at least multi-core single-package systems are the norm today, even in smartphones and laptops. 

And that is not even taking into account the fact that you're effectively benchmarking the optimization of your compiler more than your computing platform: Get Dhrystone 2.2 (from 1988!), run

 export CFLAGS=-O0
 sh ./dry.c


and get (in my case here) something like 15873016 Dhrystones per second

Running 

 export CFLAGS=-O3
 sh ./dry.c


gets you (in my case here) 43327556 Dhrystones per second – a speedup by a factor of 3. So, I couldn't have even throttled my CPU speed enough to compensate that difference!  

OK, you'll say, obviously, you'd always use the same compiler settings for benchmarking (which, then?) but consider this; instead of using GCC 9.3.1, let's use clang 9.0.1:

 export CC=clang
 export CFLAGS=-O0
 sh ./dry.c


gives me 16447368 Dhrystones/seconds, and 45126352 for -O3, so, even just switching between the two major compilers has some 10% difference.

To make things more confusing, I can bump GCC performence by another 10% using -flto, probably because it results in compacter code, utilizing CPU caches better – something that pretty much didn't even exist in 1988. clang with -Ofast -flto makes it to 64766840 Dhrystone a second. That's four times faster than without optimizations. See, Dhrystone is by no means benchmarking my CPU – it's really benchmarking my compiler (and my ability to optimize my compiler usage for this benchmark).

So, DMIP is meaningless, and your question perfectly illustrates that. It's still getting thrown around a lot – but that's for marketing reasons, not for technical ones.