Add some documentation and a new flag so that we can safely enabled using
the ofnode interface to write to the device tree.

Signed-off-by: Simon Glass <sjg@chromium.org>

At present ofnode only works with a single device tree, for the most part.
This is the control FDT used by U-Boot.

When booting an OS we may obtain a different device tree and want to
modify it. Add some initial support for this into the ofnode API.

Note that we don't permit aliases in this other device tree, since the
of_access implementation maintains a list of aliases collected at
start-up. Also, we don't need aliases to do fixups in the other FDT. So
make sure that flat tree and live tree processing are consistent in this
area.

Signed-off-by: Simon Glass <sjg@chromium.org>

The unflattening algorithm results in a single block of memory being
allocated for the whole tree. When writing new properties, these are
allocated new memory outside that block. When the block is freed, the
allocated properties remain.

Document how this works and the potential memory leak, as well as
mentioning that updating the livetree is actually supported now.

Signed-off-by: Simon Glass <sjg@chromium.org>

These docs are useful for developers, not users. Move them under that
section.

Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>