XXX: Calculate pd_cap & pd_tsk_busy_time outside pd_task_busy_time()

Rename task_busy_time() to get_task_busy_time() and pd_task_busy_time()
to get_pd_busy_time().
Calculate pd_cap in first loop of feec().

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>