ARM: ep93xx: mark most board files as unused

Most of the remaining ARM board files in the kernel have no known users,
and we plan to remove those in early 2023.

For ep93xx, Alexander Sverdlin still has access to the edb93xx family
of reference boards, while Nikita Shubin has a ts7250 and is working on
a device tree conversion for those. Hartley Sweeten has a
MACH_VISION_EP9307 that is still in use.

This is a total of nine machine definitions that we will keep
around, but these are all similar machines and are defined in only
two board files. The other six board files now have a dependency on
CONFIG_UNUSED_BOARD_FILES to indicate that they are likely going away.

Cc: Nikita Shubin <nikita.shubin@maquefel.me>
Acked-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Cc: Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>