[ARM] Kirkwood: disable propagation of mbus error to the CPU local bus

Disable propagation of mbus errors to the CPU local bus, as this causes
mbus errors (which can occur for example for PCI aborts) to throw CPU
aborts, which we're not set up to deal with.

Reported-by: Dieter Kiermaier <dk-arm-linux@gmx.de>
Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: Nicolas Pitre <nico@marvell.com>