Add a reference to snps,dw-pcie.yaml (and indirectly pci-bus.yaml) schemas.
With this, the common bus properties can be dropped from the schema.

Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: linux-pci@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210719220351.2662758-1-robh@kernel.org