samd/mphalport: Simplify mp_hal_delay_ms().

Do NOT use `mp_hal_delay_us()` for short delays.  This was initially done
to make short delays precise, but it does not allow for scheduling.  Leave
using `mp_hal_delay_us()` to user code if needed.

Signed-off-by: robert-hh <robert@hammelrath.com>

docs/samd/quickref.rst

@@ -65,6 +65,8 @@ Use the :mod:`time <time>` module::
     start = time.ticks_ms() # get millisecond counter
     delta = time.ticks_diff(time.ticks_ms(), start) # compute time difference
 
+Note that :func:`time.sleep_us()` delays by busy waiting. During that time, other tasks are
+not scheduled.
 
 Clock and time
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