openrisc: prevent VGA console, fix builds

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

   drivers/built-in.o: In function `vgacon_save_screen':
   vgacon.c:(.text+0x20e0): undefined reference to `screen_info'
   vgacon.c:(.text+0x20e8): undefined reference to `screen_info'
   drivers/built-in.o: In function `vgacon_init':
   vgacon.c:(.text+0x284c): undefined reference to `screen_info'
   vgacon.c:(.text+0x2850): undefined reference to `screen_info'
   drivers/built-in.o: In function `vgacon_startup':
   vgacon.c:(.text+0x28d8): undefined reference to `screen_info'
   drivers/built-in.o:vgacon.c:(.text+0x28f0): more undefined references to `screen_info' follow

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Chen Gang <gang.chen@asianux.com>
Cc: Jonas Bonn <jonas@southpole.se>
Signed-off-by: Stafford Horne <shorne@gmail.com>