avr32: select generic atomic64_t support

Enable use of the generic atomic64 implementation on AVR32 platforms.
Without this the kernel fails to build as the architecture does not
provide its version.

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>
Acked-by: Hans-Christian Egtvedt <egtvedt@samfundet.no>
Cc: Haavard Skinnemoen <hskinnemoen@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>