arch/mn10300/Kconfig: select CONFIG_GENERIC_ATOMIC64

mn10300 doesn't provide its own atomic64 implementation, so it should pull
in the generic one.

Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>