PCI: endpoint: Add new pci_epc_ops to get EPC features

Add a new pci_epc_ops ->get_features() to get the features
supported by the EPC. Since EPC can provide different features to
different functions, the ->get_features() ops takes _func_no_ as
an argument.

Tested-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>