cpuidle: suspend/resume scheduler tick timer during cpu idle state entry/exit

cpuidle: suspend/resume scheduler tick timer during cpu idle state entry/exit

cpuidle can collaborate with scheduler to reduce unnecessary timer
interrupt. For example, credit scheduler accounting timer
doesn't need to be active at idle time, so it can be stopped at
cpuidle entry and resumed at cpuidle exit. This patch implements this
function by adding two ops in scheduler: tick_suspend/tick_resume, and
implement them for credit scheduler

Signed-off-by: Yu Ke <ke.yu@intel.com>
Signed-off-by: Tian Kevin <kevin.tian@intel.com>