soc/intel/dnv: Add ACPI _PRT methods for virtual root ports

This eliminates Linux kernel warnings that look like:
    pcieport 0000:00:17.0: can't derive routing for PCI INT B
    ixgbe 0000:07:00.1: PCI INT B: no GSI - using ISA IRQ 10

Change-Id: I2029e7a8252b9e48c1df457d8da5adce7d1ac21d
Signed-off-by: Stephen Douthit <stephend@silicom-usa.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34713
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/src/soc/intel/denverton_ns/acpi/northcluster.asl b/src/soc/intel/denverton_ns/acpi/northcluster.asl
index 58d63c2..b102f2f 100644
--- a/src/soc/intel/denverton_ns/acpi/northcluster.asl
+++ b/src/soc/intel/denverton_ns/acpi/northcluster.asl
@@ -167,4 +167,9 @@
 // Virtual root port 2
 Device (VRP2) {
 	Name   (_ADR, 0x00060000)
+
+	Method (_PRT)
+	{
+		Return (IRQM (6))
+	}
 }