iommu/vt-d: Refine the interfaces to create IRQ for DMAR unit

Refine the interfaces to create IRQ for DMAR unit. It's a preparation
for converting DMAR IRQ to hierarchical irqdomain on x86.

It also moves dmar_alloc_hwirq()/dmar_free_hwirq() from irq_remapping.h
to dmar.h. They are not irq_remapping specific.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: David Cohen <david.a.cohen@linux.intel.com>
Cc: Sander Eikelenboom <linux@eikelenboom.it>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: iommu@lists.linux-foundation.org
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dimitri Sivanich <sivanich@sgi.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Joerg Roedel <joro@8bytes.org>
Link: http://lkml.kernel.org/r/1428905519-23704-20-git-send-email-jiang.liu@linux.intel.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>