Add pci_epc_ops to map physical address to MSI address and return MSI data.
The physical address is an address in the outbound region. This is required
to implement doorbell functionality of NTB (non-transparent bridge) wherein
EPC on either side of the interface (primary and secondary) can directly
write to the physical address (in outbound region) of the other interface
to ring doorbell using MSI.

Link: https://lore.kernel.org/r/20210201195809.7342-9-kishon@ti.comSigned-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>