nds32 does not support VGA console, so prevent that kconfig symbol from
being enabled for nds32, thus fixing these build errors:

  drivers/video/console/vgacon.o: In function `vgacon_save_screen':
  /NOBACKUP/sqa2/greentime/contrib/src_pkg/linux-nds32/drivers/video/console/vgacon.c:1327:
  undefined reference to `screen_info'
  /NOBACKUP/sqa2/greentime/contrib/src_pkg/linux-nds32/drivers/video/console/vgacon.c:1327:
  undefined reference to `screen_info'
  /NOBACKUP/sqa2/greentime/contrib/src_pkg/linux-nds32/drivers/video/console/vgacon.c:1328:
  undefined reference to `screen_info'
  /NOBACKUP/sqa2/greentime/contrib/src_pkg/linux-nds32/drivers/video/console/vgacon.c:1328:
  undefined reference to `screen_info'
  drivers/video/console/vgacon.o: In function `vgacon_init':
  /NOBACKUP/sqa2/greentime/contrib/src_pkg/linux-nds32/drivers/video/console/vgacon.c:591:
  undefined reference to `screen_info'
  drivers/video/console/vgacon.o:/NOBACKUP/sqa2/greentime/contrib/src_pkg/linux-nds32/drivers/video/console/vgacon.c:591:
  more undefined references to `screen_info' follow
  make: *** [vmlinux] Error 1
Signed-off-by: Greentime Hu <greentime@andestech.com>