idle: Implement a per-cpu idle-polling mode

cpu_idle_poll_ctrl provides a way of switching the
idle thread to use cpu_idle_poll instead of the arch
specific lower power mode callbacks (arch_cpu_idle).
cpu_idle_poll spins on a flag in a tight loop with
interrupts enabled.

In some cases it may be useful to enter the tight loop
polling mode only on a particular CPU. This allows
other CPUs to continue using the arch specific low
power mode callbacks. Provide an API that allows this.

Change-Id: I7c47c3590eb63345996a1c780faa79dbd1d9fdb4
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>