PCI: artpec6: Implement dw_pcie_ep operation get_features

PCI: artpec6: Implement dw_pcie_ep operation get_features

All non-DWC EPC drivers implement (struct pci_epc *)->ops->get_features().
All DWC EPC drivers implement (struct dw_pcie_ep *)->ops->get_features(),
except for pcie-artpec6.c.

epc_features has been required in pci-epf-test.c since commit 6613bc2301ba
("PCI: endpoint: Fix NULL pointer dereference for ->get_features()").

A follow-up commit will make further use of epc_features in EPC core code.

Implement epc_features in the only EPC driver where it is currently not
implemented.

Link: https://lore.kernel.org/r/20241213143301.4158431-12-cassel@kernel.org
Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>