examples/hwapi: Add soft_pwm example converted to uasyncio.

examples/hwapi/soft_pwm_uasyncio.py

+# See original soft_pwm.py for detailed comments.
+import uasyncio
+from hwconfig import LED
+
+
+async def pwm_cycle(led, duty, cycles):
+    duty_off = 20 - duty
+    for i in range(cycles):
+        if duty:
+            led.value(1)
+            await uasyncio.sleep_ms(duty)
+        if duty_off:
+            led.value(0)
+            await uasyncio.sleep_ms(duty_off)
+
+
+async def fade_in_out(LED):
+    while True:
+        # Fade in
+        for i in range(1, 21):
+            await pwm_cycle(LED, i, 2)
+        # Fade out
+        for i in range(20, 0, -1):
+            await pwm_cycle(LED, i, 2)
+
+
+loop = uasyncio.get_event_loop()
+loop.run_until_complete(fade_in_out(LED))