arch_topology: Add possible sibling cpu mask for cpu_topology

Commit '5ec8b591("arm64: topology: restrict updating siblings_masks
to online cpus only")' update cpu_topology sibling mask with only online
cpus. This causes issues with walt's cluster initialization code which
assumes that cpu_coregroup_mask has all possible sibling cpus. In cases
when the system is booted with limited online cpus, using maxcpu parameter,
walt's cluster initialization code will incorrectly populate less number
of cpus than physically present and doesn't have an opportunity to
correct them later.

The issue can be fixed by populating sched_cluster cpus either with
actual device-tree provided cluster cpus or with cpufreq->related_cpus.
But it is possible that, cluster cpus and cpufreq related cpus could be
different, so use device-tree populated cluster cpus as scheduler
cluster cpus.

Add possible sibling cpu support for cpu_topology.

Change-Id: I317771e85cc03fb3998457a30dae48f4f72ca546
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>