[POWERPC] XilinxFB: add of_platform bus binding

Adds the of_platform bus binding to the xilinxfb driver.  Needed to
use framebuffer devices described in the OF device tree (used by
arch/powerpc).

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Andrei Konovalov <akonovalov@ru.mvista.com>