device_pm: Update control function name and doc to indicate PM specific

PM control function is used only by the PM subsystem. Update
documentations to make it clear and name the relevant structures and
functions with _pm_ in the name.

Jira: ZEP-1044
Change-Id: I29e5b7690db34a228ed30a24a2e912e1360a0090
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>

kernel/unified/device.c

diff --git a/kernel/unified/device.c b/kernel/unified/device.c
index 44a519c..ab04ca2 100644
--- a/kernel/unified/device.c
+++ b/kernel/unified/device.c
@@ -84,11 +84,13 @@
 {
 	return 0;
 }
-int device_control_nop(struct device *unused_device,
+
+int device_pm_control_nop(struct device *unused_device,
 		       uint32_t unused_ctrl_command, void *unused_context)
 {
 	return 0;
 }
+
 void device_list_get(struct device **device_list, int *device_count)
 {