Merge branch '2022-09-29-dm-core-support-multiple-device-trees-in-ofnode' into next

To quote the author:
At present the ofnode interface is somewhat limited, in that it cannot
access the device tree provided by the OS, only the one used by U-Boot
itself (assuming these are separate). This prevents using ofnode functions
to handle device tree fixups, i.e. ft_board_setup() and the like.

The ofnode interface was introduced to allow a consistent API to access
the device tree, whether a flat tree or a live tree (OF_LIVE) is in use.

With the flat tree, adding nodes and properties at the start of the tree
(as often happens when writing to the /chosen node) requires copying a
lot of data for each operation. With live tree, such operations are
quite a bit faster, since there is no memory copying required. This has to
be weighed against the required memory allocation with OF_LIVE, as well
as the cost of unflattening and flattening the device tree which U-Boot
is running.

This series enables support for access to multiple device trees with the
ofnode interface. This is already available to some extent with OF_LIVE,
but some of the ofnode functions need changes to allow the tree to be
specified.

The mechanism works by using the top 1-4 bits of the device tree offset.
The sign bit is not affected, since negative values must be supported.

With this implemented, it becomes possible to use the ofnode interface
to do device tree fixups. The only current user is the EVT_FT_FIXUP
event.

This has two main benefits:

- ofnode can now be used everywhere, in preference to the libfdt calls
- live tree can eventually be used everywhere, with potential speed
  improvements when larger number of fixups are used

This series is only a step along the way. Firstly, while it is possible
to access the 'fix-up' tree using OF_LIVE, most of the fixup functions use
flat tree directly, rather than the ofnode interface. These need to be
updated. Also the tree must be flattened again before it is passed to the
OS. This is not currently implemented.

With OFNODE_MULTI_TREE disabled this has almost no effect on code size:
around 4 bytes if EVENT is enabled, 0 if not. With the feature enabled,
the increase is around 700 bytes, e.g. on venice2:

$ buildman -b ofn2a venice2 -sS --step 0
Summary of 2 commits for 1 boards (1 thread, 64 jobs per thread)
01: image: Drop some other #ifdefs in image-board.c
       arm:  w+   venice2
48: wip
       arm: (for 1/1 boards) all +668.0 text +668.0

This size increase is not too bad, considering the extra functionality,
but is too large to enable everywhere. So for now this features needs to
be opt-in only, based on EVENT.