dm: core: Introduce support for multiple trees

dm: core: Introduce support for multiple trees

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>