Add cpumask functions

Add a handful of cpumask functions, some of which will be used when
dealing with different PMUs on heterogeneous systems.

The maximum number of CPUs in a system, NR_CPUS, which dictates the size of
the cpumask, has been taken from the Kconfig file for each architecture,
from Linux version 5.16.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>