arch:unicore32:mm: add devmem_is_allowed() to support STRICT_DEVMEM

unicore32 supports STRICT_DEVMEM, so it needs devmem_is_allowed(), like
some of other architectures have done (e.g. arm, powerpc, x86 ...).

The related error with allmodconfig:

    CC      drivers/char/mem.o
  drivers/char/mem.c: In function ‘range_is_allowed’:
  drivers/char/mem.c:69: error: implicit declaration of function ‘devmem_is_allowed’
  make[2]: *** [drivers/char/mem.o] Error 1
  make[1]: *** [drivers/char] Error 2
  make: *** [drivers] Error 2

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>