ARC: Reset caches states variables after step

Older commit that introduced flushing of D$ and more complex logic for
invalidating I$/D$ wasn't handling single steps, it worked only when
user was "continuing". To fix this and avoid code repetition separate
function for reseting internal variables is introduced, and this
functions is called when target enter debug state.

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>