video: mb862xxfb: Require either FB_MB862XX_PCI_GDC or FB_MB862XX_LIME

The driver can be built as either a PCI or platform (OF) driver.  It
might make sense to built it as both (though that is not currently
possible) but it certainly doesn't make sense to build it as neither!
Add dependencies and add a choice group to ensure that exactly one of
FB_MB862XX_PCI_GDC and FB_MB862XX_LIME is selected.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>