framebuffer: disable vgacon on microblaze arch

Fix an allmodconfig link failer on microblaze:
  drivers/built-in.o: In function `vgacon_save_screen':
  drivers/video/console/.tmp_vgacon.o:(.text+0x8fc10):
    undefined reference to `screen_info'

Disable vgacon on microblaze because the symbol
  struct screen_info screen_info;
is not defined for the microblaze arch.

Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>