cpufreq: OMAP: add ability to lock MPU freq to user defined value during
suspend

Currently there the following scenario will prevent system from entering
suspend:
- boot system with USB device connected
- USB subsystem will set QoS constraint and lock Core to OPP100
- start system wide suspend "echo mem > sys/power/state"
- suspend will be aborted by AVS

[   82.982238] sr_classp5_suspend_noirq: smartreflex_core Calibration is
active, abort suspend

It happens because Core is locked to OPP100 by USB during boot and its
OPP50 will not be enabled until USB releases its QoS constraint. As
result, OPP50 isn't calibrated.
When USB is suspended during system wide suspend it releases QoS
constraint and CPUFreq framework will be able to switch Core to OPP50
(and it's doing this).
Therefore, when AVS sr_classp5_suspend_noirq() callback will be called -
it will detect that calibration has been started or there is a try to do
suspend with un-calibrated voltage and aborts system suspend.

In addition, the CPUFreq may request to scale MPU to High OPP at any
time during system wide suspend/resume - which may lead to system
instability or overheat, because AVS and thermal framework may be
suspended at this time.

Hence, add OMAP CPUFreq module parameter "safe_suspend_freq" which allows to
set frequency to be used during system wide suspend/resume (if set to 0
- system will be suspended using current frequency).
For example:
  echo 700000 > sys/module/omap_cpufreq/parameters/safe_suspend_freq

"safe_suspend_freq" should be selected carefully, because thermal
throttling will be inactive until freq is unlocked.

The CPUFreq lock is applied from PM_SUSPEND_PREPARE/PM_POST_SUSPEND PM
events notifiers handler.

Change-Id: I25f7ccf4ee7f44612e796cf80d8940237afe3b7c
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>