ANDROID: drivers: Introduce a legacy Energy Model loading driver

The Energy Aware Scheduler (EAS) used to rely on statically defined
Energy Models (EMs) in the device tree. Now that EAS uses the EM
framework, the old-style EMs are not usable by default.

To address this issue, introduce a driver able to read DT-based EMs and
to load them in the EM framework, hence making them available to EAS.
Since EAS now uses only the active costs of CPUs, the idle cost and
cluster cost of the old EM are ignored. The driver can be compiled in
using the CONFIG_LEGACY_ENERGY_MODEL_DT Kconfig option (off by default).

The implementation of the driver is highly inspired by the EM loading
code from android-4.14 and before (written by Robin Randhawa
<robin.randhawa@arm.com>), and the arch_topology driver (Juri Lelli
<juri.lelli@redhat.com>).

Signed-off-by: Quentin Perret <quentin.perret@arm.com>
Change-Id: I4f525dfb45113ba63f01aaf8e1e809ae6b34dd52