PCI: endpoint: Add intx_capable to epc_features struct

PCI: endpoint: Add intx_capable to epc_features struct

In struct pci_epc_features, an EPC driver can already specify if they
support MSI (by setting msi_capable) and MSI-X (by setting msix_capable).

Thus, for consistency, allow an EPC driver to specify if it supports
INTx interrupts as well (by setting intx_capable).

Since this struct is zero initialized, EPC drivers that want to claim
INTx support will need to set intx_capable to true.

Signed-off-by: Niklas Cassel <cassel@kernel.org>
[kwilczynski: add missing kernel-doc for "intx_capable"]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Link: https://lore.kernel.org/r/20250310111016.859445-13-cassel@kernel.org