irqchip/mbigen: Create irq domain for each mbigen device

For peripheral devices which connect to mbigen,mbigen is a interrupt
controller. So, we create irq domain for each mbigen device and add
mbigen irq domain into irq hierarchy structure.

Signed-off-by: Ma Jun <majun258@huawei.com>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>