drivercore: Avoid adding devices without pm_ops to dpm list

Devices without pm_ops are not required to be added in dpm list.
It also helps in improving suspend/resume latencies.

Change-Id: I19a85742fdb6f4d01363b69662ea6d7721a08b3a
Signed-off-by: Sravan Kumar Ambapuram <asravan@codeaurora.org>