arch/avr32/mm/cache.c: export symbol flush_icache_range() for module using

Need export symbol flush_icache_range() to modules, just like another
platforms have done, or can not pass compiling.

The related error (with allmodconfig under avr32):

  ERROR: "flush_icache_range" [drivers/misc/lkdtm.ko] undefined!
  make[1]: *** [__modpost] Error 1
  make: *** [modules] Error 2

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Acked-by: Hans-Christian Egtvedt <egtvedt@samfundet.no>