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>
diff --git a/dts/Kconfig b/dts/Kconfig
index 44cc6bf..deb865d 100644
--- a/dts/Kconfig
+++ b/dts/Kconfig
@@ -342,6 +342,16 @@
 	   at compilation time. This is the address of this area. It must be
 	   aligned on 2-byte boundary.
 
+config OF_TAG_MIGRATE
+	bool "Ease migration from old device trees with u-boot,dm- tags"
+	default y
+	help
+	  U-Boot moved over to use new tags to mark device tree nodes which need
+	  to be processed in SPL, before relocation, etc. Enable this option to
+	  detect old tags and handle them.
+
+	  Note: This option will be removed after the 2023.07 release.
+
 config OF_SPL_REMOVE_PROPS
 	string "List of device tree properties to drop for SPL"
 	depends on SPL_OF_CONTROL