PCI: dwc: Use devicetree 'reg[config]' to derive CPU -> ATU addr offset

The 'ranges' property of a PCI controller's parent can indicate address
translation information. Most system use 1:1 map between CPU physical and
PCI controller input addresses.

But some hardware, like i.MX8QXP, doesn't use 1:1 map.  See below diagram:

              ┌─────────┐                    ┌────────────┐
   ┌─────┐    │         │ IA: 0x8ff8_0000    │            │
   │ CPU ├───►│   ┌────►├─────────────────┐  │ PCI        │
   └─────┘    │   │     │ IA: 0x8ff0_0000 │  │            │
    CPU Addr  │   │  ┌─►├─────────────┐   │  │ Controller │
  0x7ff8_0000─┼───┘  │  │             │   │  │            │
              │      │  │             │   │  │            │   PCI Addr
  0x7ff0_0000─┼──────┘  │             │   └──► IOSpace   ─┼────────────►
              │         │             │      │            │    0
  0x7000_0000─┼────────►├─────────┐   │      │            │
              └─────────┘         │   └──────► CfgSpace  ─┼────────────►
               Bus Fabric         │          │            │    0
                                  │          │            │
                                  └──────────► MemSpace  ─┼────────────►
                          IA: 0x8000_0000    │            │  0x8000_0000
                                             └────────────┘

  bus@5f000000 {
          compatible = "simple-bus";
          #address-cells = <1>;
          #size-cells = <1>;
          ranges = <0x80000000 0x0 0x70000000 0x10000000>;

          pcie@5f010000 {
                  compatible = "fsl,imx8q-pcie";
                  reg = <0x5f010000 0x10000>, <0x8ff00000 0x80000>;
                  reg-names = "dbi", "config";
                  ...
          };
  };

Intermediate address (IA) here means the PCIe controller input address.
The pcie@5f010000 'reg[config]' address is the parent bus (PCIe controller
input) address of CfgSpace.

The ATU in MemSpace is not explicitly described via devicetree, so we
assume the offset from CPU address to intermediate MemSpace address is the
same as that for CfgSpace.

We could use bus@5f000000 'ranges' for the same purpose.

Set parent_bus_offset using dw_pcie_init_parent_bus_offset().  The
parent_bus_offset is not used yet, so no functional change intended.

Link: https://lore.kernel.org/r/20250315201548.858189-8-helgaas@kernel.org
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

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