Documentation/releases: Note the disappearance of device_t

That was truly a huge task.

Change-Id: Ifd79aaf005bf39744bd4fd930ba2441f966ec0b3
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/30669
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

Documentation/releases/coreboot-4.10-relnotes.md

@@ -10,3 +10,15 @@ notes.
  release notes for the general format.
 * The chip and board additions and removals will be updated right
 before the release, so those do not need to be added.
+
+Significant changes
+-------------------
+
+### `device_t` is no more
+coreboot used to have a data type, `device_t` that changed shape depending on
+whether it is compiled for romstage (with limited memory) or ramstage (with
+unlimited memory as far as coreboot is concerned). It's an old relic from the
+time when romstage wasn't operated in Cache-As-RAM mode, but compiled with
+our romcc compiler.
+
+That data type is now gone.