IIO: 2nd set of new device support, features and cleanup for 6.8

A late/optimistic second pull request. The bots have been poking them
since Wednesday without any issues. There are a few fixes in the
ad7091r5 driver as part of rework to enable the ad7091r8 parts that
are included at start of that series.

Includes pre-work for major changes to the DMA buffers that should
land in 6.9 and will greatly improve performance and flexibility for
high performance devices by enabling DMABUF based zero copy transfers
when we don't need to bounce the data via user space.

New device support
------------------
adi,ad7091r8
 - Major refactor of existing adi,ad7091r5 driver to separate out useful
   shared library code that can be used by I2C and SPI parts.
 - Use that library from a new driver supporting the AD7091R-2, AD7091R-4
   and AD7091R-8 12-Bit SPI ADCs.
 - Series includes some late breaking fixes for the ad7091r5.

microchip,mcp4821
 - New driver for MCP4801, MCP4802, MCP4811, MCP4812, MCP4821 and MCP4822
   I2C single / dual channel DACs.

Cleanup
-------
buffers:
 - Use IIO_SEPARATE in place of some hard-coded 0 values.
dma-buffers:
 - Simplify things to not use an outgoing queue given it only ever has
   up to two elements and we only need to track which is first.
 - Split the iio_dma_buffer_fileio_free() function up to make it easier
   to read and enable reuse in a series lining up for 6.9
iio.h
 - Drop some stale documentation of struct fields that don't exist.
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