dt-bindings: PCI: update references to Designware schema

dt-bindings: PCI: update references to Designware schema

Now that its contents were converted to a DT schema, replace
the references for the old file on existing properties.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/dfff4d94631546c53450d1baeddc694dd26b5c36.1626608375.git.mchehab+huawei@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>