PCI: dwc: Add support for EP mode

The PCIe controller dual mode is capable of operating in Root Complex
(RC) mode as well as EP mode by configuration option.

Add EP support to the DesignWare driver on top of RC mode support.

Add new property on pci_epc structure which allow to configure
pci_epf_test driver accordingly to the controller specific requirements.

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