Add PCI_EPC_IRQ_MSIX type.

Add MSI-X callbacks signatures to the ops structure.

Add sysfs interface for set/get MSI-X capability maximum number.

Update documentation accordingly.
Signed-off-by: NGustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: NKishon Vijay Abraham I <kishon@ti.com>