PCI: dwc: Add dw_pcie_parent_bus_offset()

Return the offset from CPU physical address to the parent bus address of
the specified element of the devicetree 'reg' property.

[bhelgaas: cpu_phy_addr -> cpu_phys_addr, return offset, split
.cpu_addr_fixup() checking and debug to separate patch]

Link: https://lore.kernel.org/r/20250315201548.858189-6-helgaas@kernel.org


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