frv: add atomic64_add_unless()

The build of frv allmodconfig was failing with the error:
lib/atomic64_test.c:209:9: error:

	implicit declaration of function 'atomic64_add_unless'

All the atomic64 operations were defined in frv, but
atomic64_add_unless() was not done.

Implement atomic64_add_unless() as done in other arches.

Link: http://lkml.kernel.org/r/1484781236-6698-1-git-send-email-sudipm.mukherjee@gmail.com
Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>