dt-bindings: PCI: qcom-ep: Describe optional dma-coherent property

Qualcomm SA8775P supports cache coherency on the PCIe Endpoint
controller.

Thus, allow "dma-coherent" property to be used for this device. This
fixes a part of the following error (the second part is fixed in the
next commit):

  pcie-ep@1c10000: Unevaluated properties are not allowed ('dma-coherent', 'iommus' were unexpected)

Fixes: 4b220c6 ("arm64: dts: qcom: sa8775p: Mark PCIe EP controller as cache coherent")
Signed-off-by: Dmitry Baryshkov <[email protected]>
Reviewed-by: Krzysztof Kozlowski <[email protected]>
Reviewed-by: Manivannan Sadhasivam <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <[email protected]>

Author: lumag

Documentation/devicetree/bindings/pci/qcom,pcie-ep.yaml

@@ -91,6 +91,8 @@ properties:
       - const: pcie-mem
       - const: cpu-pcie
 
+  dma-coherent: true
+
   resets:
     maxItems: 1