PCI: qcom-ep: Add Qualcomm PCIe Endpoint controller driver

Add driver for Qualcomm PCIe Endpoint controller based on the DesignWare
core with added Qualcomm-specific wrapper around the core. The driver
support is very basic such that it supports only enumeration, PCIe
read/write, and MSI. There is no ASPM and PM support for now but these will
be added later.

The driver is capable of using the PERST# and WAKE# side-band GPIOs for
operation and written on top of the DWC PCI framework.

[bhelgaas: wrap a few long lines]
Co-developed-by: Siddartha Mohanadoss <smohanad@codeaurora.org>
[mani: restructured the driver and fixed several bugs for upstream]
Link: https://lore.kernel.org/r/20210920065946.15090-3-manivannan.sadhasivam@linaro.org
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>