PCI: dwc: designware: Add EP mode support

Add endpoint mode support to designware driver. This uses the EP Core layer
introduced recently to add endpoint mode support.  *Any* function driver
can now use this designware device in order to achieve the EP
functionality.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>