timers, sched/clock: Hook into s2idle freeze path

The sched_clock() suspend/resume callbacks are called from a
syscore_ops structure that's registered after the timekeeping
syscore_ops. With the suspend-to-idle method of suspend we will
freeze the tick and timekeeping subsystem, but we don't suspend
the sched clock here, because syscore ops are not called in
suspend to idle. Hook this up directly in the timekeeping freeze
path when we go idle on the last CPU. This way, the sched clock
doesn't jump across a suspend/resume cycle.

Change-Id: I2ea488dce7186d6210d3af34b0e1a67c9f1cdf78
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>