Add LIS3DSH accelometer support to staccel.py

STM32F4 Discovery board has two different types of accelerometer.

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/user_manual/DM00039084.pdf
>Two different versions of motion sensor (U5 in schematic) are available on the board depending the PCB version. The LIS302DL is present on board MB997B (PCB revision B) and the LIS3DSH is present on board MB997C (PCB rev C).

`stmhal/boards/STM32F4DISC/staccel.py` is only for LIS302DL version boards now.
This patch add LIS3DSH support.