Add mps2-an385 (Cortex-M3) and mps2-an500 (Cortex-M7)

[mkannwischer]

Once #16 and #17 are done, we also want to add the corresponding mps2 qemu platforms, so we can perform tests in CI for these microarchitectures as well.

I hacked together some simple Makefiles for those mps2-an385 and mps-an500 that should be relatively simple to add in here.
Maybe while we are at it, we unify the current mps2-an386 with what I have here: https://github.com/mkannwischer/hw/tree/master/mps2-an386

The platform code all comes from https://github.com/ARMmbed/mbed-os.
Ideally, we want to pull in code from there, so we do not copy the code into this repo.