MLECO-3659: Improvement for NPU PMU counters

The NPU idle count could have been erraneously high as
the counters were always running. This change utilises
callback functions to start/stop the counters only when
the inferences start/stop executing on the NPU.

Changes have been made to cache maintenance functions
called from within the NPU driver's pipeline to reduce
the overhead caused by these.

Change-Id: I69db0d3b3f3fe5b2847e15b5c3096cb1e0484176
Signed-off-by: Kshitij Sisodia <kshitij.sisodia@arm.com>