sched/core: Warn if ENERGY_AWARE is enabled but data is missing

If the EAS energy model is missing or incomplete, i.e. sd_scs is NULL, then
sched_group_energy will return -EINVAL on the assumption that it raced with a
CPU hotplug event. In that case, energy_diff will return 0 and the energy-aware
wake path will silently fail to trigger any migrations.

This case can be triggered by disabling CONFIG_SCHED_MC on existing platforms,
so that there are no sched_groups with the SD_SHARE_CAP_STATES flag, so that
sd_scs is NULL.

Add checks so that a warning is printed if EAS is ever enabled while the
necessary data is not present.

Change-Id: Id233a510b5ad8b7fcecac0b1d789e730bbfc7c4a
Signed-off-by: Brendan Jackman <brendan.jackman@arm.com>