docs: iio: ad7380: add alert support

	root:~# systemctl restart iiod

Alert

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2 channels variants of the ad738x family, can use the SDOB line as an alert pin

when configured in 1 SDO line mode. 4 channels variants, can use SDOD as an

alert pin when configured in 1 or 2 SDO line(s) mode, although only 1 SDO line

mode is currently supported by the driver (see `SPI wiring modes`_).

At the end of a conversion the active-low alert pin gets asserted if the

conversion result exceeds the alert high limit or falls below the alert low

limit. It is cleared, on a falling edge of CS. The alert pin is common to all

channels.

User can enable alert using the regular iio events attribute:

.. code-block:: bash

	events/thresh_either_en

The high and low thresholds are common to all channels and can also be set using

regular iio events attributes:

.. code-block:: bash

	events/in_thresh_falling_value

	events/in_thresh_rising_value

If debugfs is available, user can read the ALERT register to determine the

faulty channel and direction.

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

Channel selection and sequencer (single-end chips only)

-------------------------------------------------------

- Rolling average oversampling

- Power down mode

- CRC indication

- Alert

Device buffers