ARM: Fix negative idle stats for offline cpu

We see negative idle stats because of cpu dying without
cleaning up idle entry statistics. When a cpu is offline,
the most immediate thing you'd want to do is just call
into cpu_die() and not waste time calling notifier(IDLE_START).

CRs-Fixed: 414554
Change-Id: Iadc6a3ca39997e0ccf65d2a29b004e24b1b211a1
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>
Signed-off-by: Taniya Das <tdas@codeaurora.org>