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>