genirq/MSI: Reorginize struct msi_desc to prepare for support of generic MSI

Reorganize struct msi_desc so it could be reused by other MSI
drivers. We have the following layout now:

struct msi_desc {
       /* Shared device/bus independent data */
       ...
       union {
       	     /* PCI specific data */
	     struct {
	     	    ...
	     };
       };
};

We need to have anonymous union and a anonymous structure for the PCI
fields, otherwise we would have to change all instances using these
fields.

For non PCI devices we will enforce a proper namespace and a non
anonymous structure.

[ tglx: Added proper comments to the structure and massaged changelog ]

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Reviewed-by: Yijing Wang <wangyijing@huawei.com>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.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: Stuart Yoder <stuart.yoder@freescale.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Alexander Gordeev <agordeev@redhat.com>
Link: http://lkml.kernel.org/r/1436428847-8886-12-git-send-email-jiang.liu@linux.intel.com


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