schemas: pci: bridge: Document "supports-d3" property

Even though the PCIe spec states that all PCIe devices should support D0 and D3
states (both D3 hot and D3 cold), PCIe bridges do not follow that in practice.

Spec r3.0, section 5.3.1:

"All Functions must support the D0 and D3 states (both D3 hot and D3 cold ).
The D1 and D2 states are optional."

Also, unlike D1 and D2, D3 support cannot be determined using the standard
Power Management Control/Status (PMCSR) Register. So the OS needs to know the
D3 support using platform specific ways. Hence, for DT based platforms, let's
add a new property called "supports-d3" that specifies that the PCI bridge is
D3 capable.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>

dtschema/schemas/pci/pci-pci-bridge.yaml

@@ -28,4 +28,10 @@ properties:
     contains:
       const: pciclass,0604
 
+  supports-d3:
+    description:
+      If present, this property specifies that the bridge supports transitioning
+      to D3 states.
+    type: boolean
+
 unevaluatedProperties: false