avr32: add generic ioremap_wc() definition in io.h

Need generic ioremap_wc(), or can not pass compiling with allmodconfig,
the related error:

    CC [M]  drivers/gpu/drm/drm_bufs.o
  drivers/gpu/drm/drm_bufs.c: In function 'drm_addmap_core':
  drivers/gpu/drm/drm_bufs.c:217: error: implicit declaration of function 'ioremap_wc'
  drivers/gpu/drm/drm_bufs.c:218: warning: assignment makes pointer from integer without a cast

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Acked-by: Hans-Christian Egtvedt <hegtvedt@cisco.com>