CodeXL 1.7 does not provide power profiling for discrete GPUs. This feature will be added very shortly in the next CodeXL release, and will support the R9 390.
The target devices available for offline compilation and static analysis are set by the driver installed on the local station. Edit: strike-out incorrect info. Catalyst 15.7 supports the Fiji family of GPUs including the R9 390. In the CodeXL Options dialog, Analyze tab, CodeXL 1.7 will display these devices under the "Graphics IP v8 (Rx 200 series)" node in the devices tree. This node actually includes devices from the R9 300 series as well. This caption will be fixed in the next CodeXL release.
Hope this helps.
CodeXL 1.8 was released yesterday Aug-20, and it supports power profiling of discrete GPUs, including the R9 390.
The target devices table is now in order with respect to Rx 300 series.