arm/arm64: Add sanity checks to the cpumask API

Make sure that any calls to the cpumask API target a valid CPU. The
CPU is identified by an int in the range [0, nr_cpus), where nr_cpus
is set based on information in the DT.

Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>