stm32: Add machine.Timer with soft timer implementation.

This commit adds an implementation of machine.Timer backed by the soft
timer mechanism.  It allows an arbitrary number of timers with 1ms
resolution, with an associated Python callback.  The Python-level API
matches existing ports that have a soft timer, and is used as:

    from machine import Timer
    t = Timer(freq=10, callback=lambda t:print(t))
    ...
    t = Timer(mode=Timer.ONE_SHOT, period=2000, callback=lambda t:print(t))
    ...
    t.deinit()

0 files changed, 0 insertions, 0 deletions