schemas: pci-bus-common: Add VIO 1.8V supply for M.2 slots

From the PCI Express M.2 specification rev. 5.1:

    The VIO 1.8 V power source is provided to supply the Adapter I/O
    buffer circuitry operating at 1.8 V sideband signaling. This power
    source is required for Adapters that support 1.8 V sideband signaling.
    Platforms that support 1.8 V sideband signaling must provide this
    power source. Adapters may provide 1.8 V sideband signaling based
    on the detection of this power source.

Add a property for describing this supply, so that it can be referenced
and controlled along with the other PCIe slot power sources.

Signed-off-by: Chen-Yu Tsai <[email protected]>

Author: wens

dtschema/schemas/pci/pci-bus-common.yaml

@@ -179,6 +179,9 @@ properties:
   vpcie3v3aux-supply:
     description: 3.3v AUX regulator phandle for the slot
 
+  vio1v8-supply:
+    description: 1.8v VIO regulator phandle for M.2 slot
+
   eq-presets-8gts:
     description:
       Contains the equalization preset values for PCIe data rates 8.0 GT/s.