ARM: mmp: mark all board files for removal

The mmp platform supports both ATAGS based board files and DT
booting, but it appears that nobody has been interested in
board files for a long time.

Mark all of them for removal in early 2023 with a dependency
on CONFIG_UNUSED_BOARD_FILES, leaving only the DT support
for the future, unless someone pops up who uses them.

Cc: Lubomir Rintel <lkundrak@v3.sk>
Cc: Haojian Zhuang <haojian.zhuang@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>