arch: unicore32: ksyms: export '__cpuc_coherent_kern_range' to avoid compiling failure

flush_icache_range() is '__cpuc_coherent_kern_range' under unicore32,
and lkdtm.ko needs it. At present, '__cpuc_coherent_kern_range' is
still used by unicore32, so export it to avoid compiling failure.

The related error (with allmodconfig under unicore32):

  ERROR: "__cpuc_coherent_kern_range" [drivers/misc/lkdtm.ko] undefined!

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Acked-by: Xuetao Guan <gxt@mprc.pku.edu.cn>
Signed-off-by: Xuetao Guan <gxt@mprc.pku.edu.cn>