irqdomain: Introduce helper function irq_domain_add_hierarchy()

irqdomain: Introduce helper function irq_domain_add_hierarchy()

Introduce helper function irq_domain_add_hierarchy(), which creates
a linear irqdomain if parameter 'size' is not zero, otherwise creates
a tree irqdomain.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Yijing Wang <wangyijing@huawei.com>
Cc: Yingjoe Chen <yingjoe.chen@mediatek.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Link: http://lkml.kernel.org/r/1416061447-9472-5-git-send-email-jiang.liu@linux.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>