avoid adding devices without pm_ops to dpm list

[Sravan <asravan@codeaurora.org>]

Hello,

I have come across below code which looks can be optimized to reduce 
suspend/resume latencies.

During device registration at boot up, device_add() is called.
In this device_add() function, device_pm_add() is called without 
checking if the device has pm_ops or not.
Thus dpm_list is generated with all devices even if some devices don't 
have pm_ops.

device_add() -> device_pm_add()-> generate dpm_list

During suspend/resume process, suspend/resume APIs are called for 
devices present in dpm_list.
due to this, suspend/resume APIs for devices without pm_ops also called 
and just return with NULL.

Can we optimize this code by adding *ONLY* devices with pm_ops to 
dpm_list and ignore remaining?
It will help reduce suspend/resume latencies. I observed 10mS savings 
with my debug code. It may change based on number of devices without pm_ops.

Let me know your opinion/suggestions on this.

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