PCI: dwc: Use custom pci_ops for root bus DBI vs ECAM config access

When the vendor configuration space is 256MB aligned, the DesignWare PCIe
host driver enables ECAM access and sets the DBI base to the start of the
config space. This causes vendor drivers to incorrectly program iATU
regions, as they rely on the DBI address for internal accesses.

To fix this, avoid overwriting the DBI base when ECAM is enabled.  Instead,
introduce a custom pci_ops that accesses the DBI region directly for the
root bus and uses ECAM for other buses.

Fixes: f6fd357f ("PCI: dwc: Prepare the driver for enabling ECAM mechanism using iATU 'CFG Shift Feature'")
Reported-by: Ron Economos <re@w6rz.net>
Closes: https://lore.kernel.org/all/eac81c57-1164-4d74-a1b4-6f353c577731@w6rz.net/


Suggested-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
[bhelgaas: commit log]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Ron Economos <re@w6rz.net>
Link: https://patch.msgid.link/20251017-ecam_fix-v1-1-f6faa3d0edf3@oss.qualcomm.com