mcountinhibit simplification

Created by: halfdan-dolva

Simplified mcountinhibit register implementation to make mcountinhibit_n signal show true write value.

This is a fix for #347 (closed), perf_counters_instructions test passes on strichmo/pr/mcountinhibit fork with this change.

SEC clean change. Slight area improvement (less than noise margin).

Signed-off-by: Halfdan Bechmann halfdan.bechmann@silabs.com