IIO: 3rd set for 6.9 - cleanup.h related.

I have separated this set out from the more normal patches as they can
go separately and that may simplify the merge window.  Greg, up to you
how you wish to handle this in the char-misc tree.

Introduces __free() based handling for fwnode_handle_put() to
allow scope based release of these handles on early exit from functions.

Also introduced device_for_each_child_node_scoped() to provide a
a convenient way to process child nodes without the need to explicitly
handle the fwnode_handle_put() needed on early exits from the loop.
Typically these early exits are a result of error handling or completion
of a search and have proven very prone to being missed.

One instance of such a leaked resource was found during these conversions
though review of that patch was too late for this pull request.

A number of drivers are also converted over to generic fwnode handling from
the device tree specific version.
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