sched: hmp: fix out-of-range CPU possible

If someone hotplugs all the little CPUs while another CPU is handling
a wakeup, we can potentially return new_cpu == NR_CPUS from
hmp_select_slower_cpu (which is called internally by
hmp_best_little_cpu as well). We will use this to deref the
per_cpu rq array in hmp_next_down_delay which can go boom.

Signed-off-by: Chris Redpath <chris.redpath@arm.com>
arm64: Enable Heterogenous Multi Processing by default

HMP needs more tuning but its usefulness has been proved
so enable it by default. There might still be corner cases
where issues will crop up, but for demos and evaluation
is should be ready to use.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>