drivers: cpuidle: lpm-levels: Support s2idle

If we support suspend-to-idle we can avoid the overhead in
hotplugging out CPUs and tearing things down during suspend. Most
things are already in place to support this so we can pretty much
re-use the platform suspend ops to enter the s2idle state for
suspending with cpuidle.

Change-Id: Ib062d93a0c122a644af4a729dbb202b88c9a6c56
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>