arm64: factor out energy model from topology shim layer

To be able to support multiple energy models before we have the
full-fletched dt solution in arm64 (e.g. for platform Arm Juno and
Hisilicon Hikey) factor out the static energy model data and the
appropriate access function into energy_model.h.

The patch uses of_match_node() to match the compatible string with the
appropriate platform energy model data, i.e. the patch introduces a
dependency to CONFIG_OF_FLATTREE for propagating the energy model data
towards the task scheduler.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>