stats: Change the way topology returns nodes for "all"
stats: Change the way topology returns nodes for "all"

The "all" level previously returned a flat list of nodes, whereas
this is intended to show a system-wide topology and now returns
[[node_1, node_2, .. node_n]]

Change-Id: I908fe4b24482cad47d58d595a9bb2fd6477c8e85
Signed-off-by: Kapileshwar Singh <d373e2b6407ea84be359ce4a11e8631121819e79@arm.com>

derkling/trappy,JaviMerino/trappy,sinkap/trappy,joelagnel/trappy,sinkap/trappy,bjackman/trappy,ARM-software/trappy,ARM-software/trappy,joelagnel/trappy,joelagnel/trappy,ARM-software/trappy,sinkap/trappy,joelagnel/trappy,bjackman/trappy,JaviMerino/trappy,bjackman/trappy,sinkap/trappy,ARM-software/trappy,bjackman/trappy,derkling/trappy,derkling/trappy,JaviMerino/trappy