Add test managed wrappers for of_overlay_apply() that automatically
removes the overlay when the test is finished. This API is intended for
use by KUnit tests that test code which relies on 'struct device_node's
and of_*() APIs.

KUnit tests will call of_overlay_apply_kunit() to load an overlay that's
been built into the kernel image. When the test is complete, the overlay
will be removed.

This has a few benefits:

 1) It keeps the tests hermetic because the overlay is removed when the
    test is complete. Tests won't even be aware that an overlay was
    loaded in another test.

 2) The overlay code can live right next to the unit test that loads it.
    The overlay and the unit test can be compiled into one kernel module
    if desired.

 3) We can test different device tree configurations by loading
    different overlays. The overlays can be written for a specific test,
    and there can be many of them loaded per-test without needing to jam
    all possible combinations into one DTB.

 4) It also allows KUnit to test device tree dependent code on any
    architecture, not just UML. This allows KUnit tests to test
    architecture specific device tree code.

There are some potential pitfalls though. Test authors need to be
careful to not overwrite properties in the live tree. The easiest way to
do this is to add and remove nodes with a 'kunit-' prefix, almost
guaranteeing that the same node won't be present in the tree loaded at
boot.
Suggested-by: Rob Herring <robh@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Saravana Kannan <saravanak@google.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20240718210513.3801024-3-sboyd@kernel.org