PCI: qcom: Add IPQ60xx support

IPQ60xx series of SoCs have one port of PCIe gen 3. Add support for that
platform.

The code is based on downstream[1] Codeaurora kernel v5.4 (branch
win.linuxopenwrt.2.0).

Split out the DBI registers access part from .init into .post_init. DBI
registers are only accessible after phy_power_on().

[1] https://source.codeaurora.org/quic/qsdk/oss/kernel/linux-ipq-5.4/

Link: https://lore.kernel.org/r/f7f848653c99abbf9a0f877949a44e52329543ae.1655799816.git.baruch@tkos.co.il
Tested-by: Robert Marko <robert.marko@sartura.hr>
Signed-off-by: Selvam Sathappan Periakaruppan <quic_speriaka@quicinc.com>
Signed-off-by: Baruch Siach <baruch.siach@siklu.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Johan Hovold <johan+linaro@kernel.org>
Acked-by: Stanimir Varbanov <svarbanov@mm-sol.com>