The DWC glue drivers requiring an active reference clock from the PCIe host
for initializing their PCIe EP core, set a flag called 'core_init_notifier'
to let DWC driver know that these drivers need a special attention during
initialization. In these drivers, access to the hw registers (like DBI)
before receiving the active refclk from host will result in access failure
and also could cause a whole system hang.

But the current DWC EP driver doesn't honor the requirements of the drivers
setting 'core_init_notifier' flag and tries to access the DBI registers
during dw_pcie_ep_init(). This causes the system hang for glue drivers such
as Tegra194 and Qcom EP as they depend on refclk from host and have set the
above mentioned flag.

To workaround this issue, users of the affected platforms have to maintain
the dependency with the PCIe host by booting the PCIe EP after host boot.
But this won't provide a good user experience, since PCIe EP is _one_ of
the features of those platforms and it doesn't make sense to delay the
whole platform booting due to PCIe requiring active refclk.

So to fix this issue, let's move all the DBI access from
dw_pcie_ep_init() in the DWC EP driver to the dw_pcie_ep_init_complete()
API. This API will only be called by the drivers setting
'core_init_notifier' flag once refclk is received from host. For the rest
of the drivers that gets the refclk locally, this API will be called
within dw_pcie_ep_init().

Link: https://lore.kernel.org/linux-pci/20240327-pci-dbi-rework-v12-1-082625472414@linaro.org
Fixes: e966f739 ("PCI: dwc: Refactor core initialization code for EP mode")
Co-developed-by: Vidya Sagar <vidyas@nvidia.com>
Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>