Add .frequency property to BusIO's I2C

Despite being a kwarg within the constructor (https://circuitpython.readthedocs.io/en/4.x/shared-bindings/busio/I2C.html#busio.I2C), Frequency is not a set-able property.

Would it be possible to set the frequency via a call to i2c.Frequency?

>>> i2c.
deinit          readfrom_into   scan            try_lock
unlock          writeto

Currently, I need to de-init the i2c object, change the frequency, then re-init it with a new frequency.

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