PCI: dwc: Prepare the driver for enabling ECAM mechanism using iATU 'CFG Shift Feature'

In order to enable PCIe ECAM mechanism in DWC driver as per the 'CFG Shift
Feature' documented in Designware databook r5.20a, sec 3.10.10.3, prepare
the driver to handle the one time iATU setup and creating ECAM window.

Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
[mani: splitted the preparatory code into a separate commit for bisectability]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20250923-controller-dwc-ecam-v10-2-e84390ba75fa@kernel.org