fbdev/xilinxfb: Microblaze driver support

DCR bus is natively used for PowerPC. Microblaze has
no infrastructure for compile DCR that's why is necessary
to exclude it.

Signed-off-by: Michal Simek <monstr@monstr.eu>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
CC: Andrei Konovalov <akonovalov@ru.mvista.com>
CC: "David S. Miller" <davem@davemloft.net>
CC: linux-kernel@vger.kernel.org