iio: events.h: add event identifier macros for differential channel

Currently, there are 3 helper macros in iio/events.h to create event
identifiers:
- IIO_EVENT_CODE : create generic event identifier for differential and non
  differential channels
- IIO_MOD_EVENT_CODE : create event identifier for modified (non
  differential) channels
- IIO_UNMOD_EVENT_CODE : create event identifier for unmodified (non
  differential) channels

For differential channels, drivers are expected to use IIO_EVENT_CODE.
However, only one driver in drivers/iio currently uses it correctly,
leading to inconsistent event identifiers for differential channels that
don’t match the intended attributes (such as max1363.c that supports
differential channels, but only uses IIO_UNMOD_EVENT_CODE).

To prevent such issues in future drivers, a new helper macro,
IIO_DIFF_EVENT_CODE, is introduced to specifically create event identifiers
for differential channels. Only one helper is needed for differential
channels since they cannot have modifiers.

Additionally, the descriptions for IIO_MOD_EVENT_CODE and
IIO_UNMOD_EVENT_CODE have been updated to clarify that they are intended
for non-differential channels,

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241028-iio-add-macro-for-even-identifier-for-differential-channels-v1-1-b452c90f7ea6@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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