openrisc: prevent VGA console, fix builds

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 <[email protected]>
Reported-by: kbuild test robot <[email protected]>
Cc: Chen Gang <[email protected]>
Cc: Jonas Bonn <[email protected]>
Signed-off-by: Stafford Horne <[email protected]>