pios l3gd20: add support for downsampling

This adds down sampling support to the l3gd20 driver.
This is another try to fix #371.
The rationale behind this is that a processor reset is not able to clear the sensor's fifos. As this driver only read one sample per data ready interrupt it would lag behind by <fifo_size> samples after a processor reset.
I was able to confirm this behavior by looking at some temporarily added counters in the debugger.

So please do some very careful testing to tell me if this fixes the erroneous behavior.