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to see what features they may support before calling identify/probe/attach. This is necessary because the ACPI 3.0 spec requires driver support be advertised before running any methods. For now, the flags are as specified in for the _PDC and _OSC methods but we can support private flags as needed. Add an implementation of this for acpi_cpu. It checks all its children (notably cpufreq drivers) and calls the _PDC method to report the results.