dm: Add support for handling old u-boot,dm- tags

dm: Add support for handling old u-boot,dm- tags

Add a CONFIG option to deal with this automatically, printing a warning
when U-Boot starts up. This can be useful if the device tree comes from
another project.

We will maintain this through the 2023.07 release, providing 6 months
for people to notice.

Signed-off-by: Simon Glass <sjg@chromium.org>
Version 4:
Acked-by: Michal Simek <michal.simek@amd.com>