dm: Remove pre-schema tag support

dm: Remove pre-schema tag support

Support for using "u-boot,dm-..." rather than "bootph-..." has been
deprecated since February 2023. Any platforms using this have had a
console message saying to migrate by 2023.07. Go and remove all support
here now, for the v2026.01 release.

The results of this change that aren't clear from the above are that we
still have a checkpatch.pl error message, and document in
doc/develop/spl.rst that they have been migrated since 2023. We also
change the key2dtsi.py tool to use the correct bootph phase rather than
the legacy phase.

Signed-off-by: Tom Rini <trini@konsulko.com>