dt-bindings:iio:potentiometer: give microchip,mcp4531 its own binding

We use this part in an example for the envelope detector. That showed
that we need to allow for the #io-channel-cells property which
trivial-devices.yaml does not.

It doesn't make sense to add that property to trivial-devices as
it only applies for those devices that can provide some sort of
DAC or ADC service to another device driver.  Hence solution will
be to pull some IIO devices out to have their own file on a case
by case basis.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Peter Rosin <peda@axentia.se>
Link: https://lore.kernel.org/r/20201031181242.742301-4-jic23@kernel.org