Makefile: let 'make clean' remove files created by DEVICE_TREE_DEBUG

Originally, 'make DEVICE_TREE_DEBUG=1' only generated extra output on
the console. But since 6cdd7597a2f ("kbuild: Produce diff between base
DT and U-Boot augmented DT if DEVICE_TREE_DEBUG=1"), that has also
left behind lots of files that clutter the repo and are hard to get
rid of manually.

Make sure that a 'make clean' will remove those files, and add them to
.gitignore.

Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>