PCI: dwc: Support ECAM mechanism by enabling iATU 'CFG Shift Feature'

Designware databook r5.20a, sec 3.10.10.3 documents the 'CFG Shift Feature'
of the internal Address Translation Unit (iATU). When this feature is
enabled, it shifts/maps the BDF contained in the bits [27:12] of the target
address in MEM TLP to become BDF of the CFG TLP. This essentially
implements the Enhanced Configuration Address Mapping (ECAM) mechanism as
defined in PCIe r6.0, sec 7.2.2.

Currently, the driver is not making use of this CFG shift feature, thereby
creating the iATU outbound map for each config access to the devices,
causing latency and wasting CPU cycles.

So to avoid this, configure the controller to enable CFG shift feature by
enabling the 'CFG Shift' bit of the 'iATU Control 2 Register'.

As a result of enabling CFG shift (ECAM), there is no longer a need to map
the DBI register space separately as the DBI region falls under the
'config' space used for ECAM (as DBI is used to access the Root Port).

For enabling ECAM using CFG shift, the platform has to satisfy following
requirements:

  1. Size of the 'config' memory space to be used as ECAM memory should be
     able to accommodate the number of buses defined in the 'bus-range'
     property of the host bridge DT node.

  2. The 'config' memory space should be 256 MiB aligned. This requirement
     comes from PCIe r6.0, sec 7.2.2, which says the base address of ECAM
     memory should be aligned to a 2^(n+20) byte address boundary. For the
     DWC cores, n is 8, so this results in 2^28 byte alignment requirement.

It should be noted that some DWC vendor glue drivers like pcie-al may use
their own ECAM mechanism. For those controllers, set
'dw_pcie_rp::native_ecam' flag and skip enabling the CFG Shift feature in
the DWC core.

Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
[mani: code split, reworded subject/description, comment, native_ecam flag]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20250923-controller-dwc-ecam-v10-4-e84390ba75fa@kernel.org

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