checks: Allow PCI bridge child nodes without an address

Some PCI bridge nodes have child nodes such as an interrupt controller
which are not PCI devices. Allow these nodes which don't have a
unit-address.

Signed-off-by: Rob Herring <robh@kernel.org>
Message-Id: <20200928201942.3242124-1-robh@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>