microblaze: Set the default irq_domain

Register the irq_domain created during initialization as the default so
that device drivers can pass NULL to irq_create_mapping and get a
virtual irq to pass to request_irq.
Signed-off-by: Dan Christensen <opello@opello.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/kernel/intc.c

@@ -172,4 +172,6 @@ void __init init_IRQ(void)
 	 * and commits this patch.  ~~gcl */
 	root_domain = irq_domain_add_linear(intc, nr_irq, &xintc_irq_domain_ops,
 							(void *)intr_mask);
+
+	irq_set_default_host(root_domain);
 }