PCI: dwc: Add ->get_features() callback function to dw_pcie_ep_ops

Each platform using Designware PCIe core can support different set of
endpoint features. Add a new callback function ->get_features() in
dw_pcie_ep_ops so that each platform using Designware PCIe core can
advertise its supported features to the endpoint function driver.

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>