Add support for MultiTech mDot module

Module is based around an STM32F405RG processor.  Let me know if I'm missing anything.
The issue I ran into here is that right now I'm intentionally underclocking the processor, mainly because it may be used in battery powered applications, and the code to set the PLL's is target specific.  So, I didn't want to specify the processor target because that won't match if someone else wants a board with this processor.  For now, I just moved the define to targets.py.  I'm assuming that maybe v3 of mbed will fix this by allowing configurable clocks?