irqchip/mbigen: Introduce mbigen_of_create_domain()

Introduce mbigen_of_create_domain() to consolidate OF related
code and prepare for ACPI later, no funtional change.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Reviewed-by: Ma Jun <majun258@huawei.com>
Tested-by: Ming Lei <ming.lei@canonical.com>
Tested-by: Wei Xu <xuwei5@hisilicon.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>