AVR32: Make cacheflush cpu-dependent

The AT32UC3A series of processors doesn't contain any cache, and issuing
cache control instructions on those will cause an exception. This commit
makes cacheflush.h arch-dependent in preparation for the AT32UC3A-support.

Signed-off-by: Gunnar Rangoy <gunnar@rangoy.com>
Signed-off-by: Paul Driveklepp <pauldriveklepp@gmail.com>
Signed-off-by: Olav Morken <olavmrk@gmail.com>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>