bus: mhi: Add MHI PCI support for WWAN modems

This is a generic MHI-over-PCI controller driver for MHI only devices
such as QCOM modems. For now it supports registering of Qualcomm SDX55
based PCIe modules. The MHI channels have been extracted from mhi
downstream driver.

This driver is for MHI-only devices which have all functionalities
exposed through MHI channels and accessed by the corresponding MHI
device drivers (no out-of-band communication).

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Bhaumik Bhatt <bbhatt@codeaurora.org>
Reviewed-by: Hemant Kumar <hemantk@codeaurora.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
[mani: fixed up the Makefile rule]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>