genirq/msi: Consolidate MSI descriptor data

All non PCI/MSI usage variants have data structures in struct msi_desc with
only one member: xxx_index. PCI/MSI has a entry_nr member.

Add a common msi_index member to struct msi_desc so all implementations can
share it which allows further consolidation.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20211210221814.350967317@linutronix.de

include/linux/msi.h

@@ -143,6 +143,7 @@ struct ti_sci_inta_msi_desc {
  *			address or data changes
  * @write_msi_msg_data:	Data parameter for the callback.
  *
+ * @msi_index:	Index of the msi descriptor
  * @pci:	[PCI]	    PCI speficic msi descriptor data
  * @platform:	[platform]  Platform device specific msi descriptor data
  * @fsl_mc:	[fsl-mc]    FSL MC device specific msi descriptor data
@@ -163,6 +164,7 @@ struct msi_desc {
 	void (*write_msi_msg)(struct msi_desc *entry, void *data);
 	void *write_msi_msg_data;
 
+	u16				msi_index;
 	union {
 		struct pci_msi_desc		pci;
 		struct platform_msi_desc	platform;