PCI: endpoint: Remove features member in struct pci_epc

Since EPC features are now implemented using pci_epc_features and
all the EPC drivers are moved to using pci_epc_features, remove
features member in struct pci_epc and all the helper macros for
configuring the features.

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>