misc: lan966x_pci: Fix dtc warn 'Missing interrupt-parent'

dtc generates the following warning when building the LAN966x devicetree
overlay (lan966x_pci.dtso):
  Warning (interrupts_property): /fragment@0/__overlay__/pci-ep-bus@0/oic@e00c0120: Missing interrupt-parent

The oic interrupt parent is the PCI device itself. The PCI device node
is the node on which the dtbo will be applied and this node already has
properties needed.

In order to remove the warning, add the missing properties in the
overlay fragment node. Properties in this node will not be added when
the overlay is applied (which is what we expect) but makes dtc happy.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/all/20241025110919.64b1cffb@canb.auug.org.au/
Fixes: 185686b ("misc: Add support for LAN966x PCI device")
Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20241029084338.194942-3-herve.codina@bootlin.com
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

Author: hcodina

drivers/misc/lan966x_pci.dtso

@@ -15,6 +15,16 @@
 / {
 	fragment@0 {
 		target-path = "";
+
+		/*
+		 * These properties allow to avoid a dtc warnings.
+		 * The real interrupt controller is the PCI device itself. It
+		 * is the node on which the device tree overlay will be applied.
+		 * This node has those properties.
+		 */
+		#interrupt-cells = <1>;
+		interrupt-controller;
+
 		__overlay__ {
 			#address-cells = <3>;
 			#size-cells = <2>;