mimxrt1011: pwmio: Enable basic PWMOut functionality

After this change, the following program works for me on the MIMXRT1010-EVK:
```python
import pwmio
import board

p = pwmio.PWMOut(board.D13, frequency=1_000_000, variable_frequency=True)
p.duty_cycle = 32868

while True:
    pass
```

Querying and varying the duty_cycle and frequency work as well.

The lowest frequency obtainable is about 2kHz; there is an additional
divider which would allow lower PWM frequencies (I think 1kHz is important
for servos?)

Something odd happens with very low duty cycles, such as
```python
>>> p.frequency = 2000
>>> p.duty_cycle = 2
```
instead of a symmetrical waveform, it's asymmetrical.  With `duty_cycle=4`,
the effect disappears.  The reason for this is probably hidden in the
datasheet, but could affect servos or other things that count pulse
widths.

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