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

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

It contains two separate schemas on it:

- snps,dw-pcie
  This one uses the pci-bus.yaml schema;
- snps,dw-pcie-ep
  This one uses the pci-ep.yaml schema.

As the:
	AllOf:
	  - $ref: <foo>

for the endpoint part is different than the PCI one, place
it on a separate yaml file.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/26025b256232c2e4bd91954907b9d92db27199a3.1626608375.git.mchehab+huawei@kernel.org


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