dt-bindings: PCI: intel,lgm-pcie: Add 'atu' resource

dt-bindings: PCI: intel,lgm-pcie: Add 'atu' resource

The 'atu' information is already set in the dwc core, if it is specified in
the devicetree. The driver uses its own default, if not set in the
devicetree. This information is hardware-specific and should therefore be
maintained in the devicetree rather than in the source.

To be backward compatible, this field is not mandatory. If 'atu' resource
is not specified in the devicetree, the driver’s default value is used.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20260417-pcie-intel-gw-v5-7-0a2b933fe04f@dev.tdt.de