dt-bindings: PCI: add snps,dw-pcie.yaml

Currently, the designware schema is defined on a text file:
	designware-pcie.txt

Convert the pci-bus part into a schema.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/53363a7609176ca56c47ef57287466ee84087dc5.1626608375.git.mchehab+huawei@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>