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

[wens]

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.

[robherring]

We already have cases of "vpcie1v8-supply", can we use that instead?

[wens]

That was my initial choice, but then it's not really a part of the core PCIe standard, so it would be a bit misleading.

I'm fine with changing it though. The description is pretty clear on what this is supposed to be.