iio: adc: ad7380: add alert support

The alert functionality is an out of range indicator and can be used as
an early indicator of an out of bounds conversion result.

ALERT_LOW_THRESHOLD and ALERT_HIGH_THRESHOLD registers are common to all
channels.

When using 1 SDO line (only mode supported by the driver right now), i.e
data outputs only on SDOA, SDOB (or SDOD for 4 channels variants) is
used as an alert pin. The alert pin is updated at the end of the
conversion (set to low if an alert occurs) and is cleared on a falling
edge of CS.

The ALERT register contains information about the exact alert status:
channel and direction. ALERT register can be accessed using debugfs if
enabled.

User can set high/low thresholds and enable alert detection using the
regular iio events attributes:

  events/in_thresh_falling_value events/in_thresh_rising_value
  events/thresh_either_en

In most use cases, user will hardwire the alert pin to trigger a shutdown.

In theory, we could generate userspace IIO events for alerts, but this
is not implemented yet for several reasons [1]. This can be implemented
later if a real use case actually requires it.

Signed-off-by: Julien Stephan <jstephan@baylibre.com>

[1] https://lore.kernel.org/all/4be16272-5197-4fa1-918c-c4cdfcaee02e@baylibre.com/

Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250108-ad7380-add-alert-support-v4-4-1751802471ba@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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