arch/unicore32/kernel/setup.c: add generic 'screen_info' to avoid compiling failure

Add generic 'screen_info' just like another architectures have done
(e.g. tile, sh, score, ia64, hexagon, and cris).

The related error (with allmodconfig under unicore32):

    LD      init/built-in.o
  drivers/built-in.o: In function `vgacon_save_screen':
  powercap_sys.c:(.text+0x21788): undefined reference to `screen_info'
  drivers/built-in.o: In function `vgacon_resize':
  powercap_sys.c:(.text+0x21b54): undefined reference to `screen_info'
  drivers/built-in.o: In function `vgacon_switch':
  powercap_sys.c:(.text+0x21cb4): undefined reference to `screen_info'
  drivers/built-in.o: In function `vgacon_init':
  powercap_sys.c:(.text+0x2296c): undefined reference to `screen_info'
  drivers/built-in.o: In function `vgacon_startup':
  powercap_sys.c:(.text+0x22e80): undefined reference to `screen_info'

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>