PCI: of: Reduce severity of missing of_root error message

PCI: of: Reduce severity of missing of_root error message

Arm64 kernels are frequently built dual ACPI/DT, and then boot in ACPI
mode. In this case, there won't be an of_root, except for rare DT described
PCIe boards.

As a result, users in the common case see this high priority worrying
message, despite the machine working as expected.

Reduce this message to pr_debug() to avoid unnecessary noise.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20260309045930.21531-1-jeremy.linton@arm.com