iio: core: register chardev only if needed

We only need a chardev if we need to support buffers and/or events.

With this change, a chardev will be created only if an IIO buffer is
attached OR an event_interface is configured.

Otherwise, no chardev will be created, and the IIO device will get
registered with the 'device_add()' call.

Quite a lot of IIO devices don't really need a chardev, so this is a minor
improvement to the IIO core, as the IIO device will take up (slightly)
fewer resources.

In order to not create a chardev, we mostly just need to not initialize the
indio_dev->dev.devt field. If that is un-initialized, cdev_device_add()
behaves like device_add().

This change has a small chance of breaking some userspace ABI, because it
removes un-needed chardevs. While these chardevs (that are being removed)
have always been unusable, it is likely that some scripts may check their
existence (for whatever logic).
And we also hope that before opening these chardevs, userspace would have
already checked for some pre-conditions to make sure that opening these
chardevs makes sense.
For the most part, there is also the hope that it would be easier to change
userspace code than revert this. But in the case that reverting this is
required, it should be easy enough to do it.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210215104043.91251-9-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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