PCI: endpoint: Add PCI Endpoint function driver for MHI bus

PCI: endpoint: Add PCI Endpoint function driver for MHI bus

Add PCI Endpoint driver for the Qualcomm MHI (Modem Host Interface) bus.

The driver implements the MHI function over PCI in the endpoint device such
as SDX55 modem. The MHI endpoint function driver acts as a controller
driver for the MHI Endpoint stack and carries out all PCI related
functionality.

Link: https://lore.kernel.org/r/20230602114756.36586-9-manivannan.sadhasivam@linaro.org
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Kishon Vijay Abraham I <kishon@kernel.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>