ARM: davinci: mark all ATAGS board files as unused

From an earlier discussion, it appears that the davinci da8xx machines
that are still functional have already been converted to DT, while the
remaining board files are only kept because nobody has stepped up to
remove them.

Mark all these boards as 'depends on UNUSED_BOARD_FILES' with the
plan to remove them in early 2023 after the next longterm supported
kernel is out.

Cc: Sekhar Nori <nsekhar@ti.com>
Cc: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>