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

                   Endpoint
  ┌───────────────────────────────────────────────┐
  │                             pcie-ep@5f010000  │
  │                             ┌────────────────┐│
  │                             │   Endpoint     ││
  │                             │   PCIe         ││
  │                             │   Controller   ││
  │           bus@5f000000      │             ┌────────►
  │           ┌──────────┐      │             │  ││dynamically
  │           │          │ Outbound Transfer  │  ││allocated
  │┌─────┐    │  Bus     ┼─────►│ ATU  ───────┘  ││PCI Addr
  ││     │    │  Fabric  │Bus   │                ││
  ││ CPU ├───►│          │Addr  │                ││
  ││     │CPU │          │0x8000_0000            ││
  │└─────┘Addr└──────────┘      │                ││
  │       0x7000_0000           └────────────────┘│
  └───────────────────────────────────────────────┘

  bus@5f000000 {
          compatible = "simple-bus";
          ranges = <0x80000000 0x0 0x70000000 0x10000000>;

          pcie-ep@5f010000 {
                  reg = <0x80000000 0x10000000>;
                  reg-names ="addr_space";
                  ...
          };
          ...
  };

In the diagram above, CPU writes data to outbound window address
0x7000_0000, and the bus fabric maps it to 0x8000_0000. The ATU uses
bus address 0x8000_0000 as input address and maps to some PCI address
dynamically allocated by a PCI device driver on the host side.

The pcie-ep@5f010000 'reg[addr_space]' is the parent bus address, which is
the input of PCIe controller, including the ATU.

Set parent_bus_offset, the offset from the CPU address to the PCIe
controller input address using dw_pcie_init_parent_bus_offset().  The
parent_bus_offset is not used yet, so no functional change intended.

[bhelgaas: commit log]
Link: https://lore.kernel.org/r/20250315201548.858189-11-helgaas@kernel.org


Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>