acpi_powerres: Interface for getting D-state of devices from userspace

[obiwac]

Summary

Currently, there's no way to do this on FreeBSD.

TODO:

• Need to make sure that the ACPI_STATE_Dx constants are actually available to userspace.
• Would like to base this on actual D-state getting instead of what our buggy D-state code things the current device D-state is.

Sponsored by: The FreeBSD Foundation

Test Plan

I will eventually update devinfo(8) with a switch to print out the D-state of ACPI devices, which will use this interface.