stmhal: Turn off DMA clocks when idle for 100 msec

Turning on each DMA block increases the current consumption
by about 8 mA. This code adds an idle timer for each DMA
block and turns off the clocks when no streams are in use
for 128 msec. Having a small timeout allows for improved
performance when back-to-back transfers are being performed.

The 128 msec is basically a guess.

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