PM / devfreq: Simplify the sysfs name of devfreq-event device

This patch just removes '.' character from the sysfs name of devfreq-event
device as following. Usually, the subsystem uses the similiar naming style
such as {framework name}{Number}.
- old : /sys/class/devfreq-event/event.(X)
- new : /sys/class/devfreq-event/event(X)

And this patch initializes the value of 'event_no' with -1
in order to remove the unneeded operation (-1) when calling
the atomic_inc_return(&event_no).

Lastly, this patch adds the ABI document for devfreq-event class.

Signed-off-by: Chanwoo Choi <[e-mail]>
Signed-off-by: MyungJoo Ham <[e-mail]>

Documentation/ABI/testing/sysfs-class-devfreq-event

···
+What:		/sys/class/devfreq-event/event(x)/
+Date:		January 2017
+Contact:	Chanwoo Choi <[e-mail]>
+Description:
+		Provide a place in sysfs for the devfreq-event objects.
+		This allows accessing various devfreq-event specific variables.
+		The name of devfreq-event object denoted as 'event(x)' which
+		includes the unique number of 'x' for each devfreq-event object.
+
+What:		/sys/class/devfreq-event/event(x)/name
+Date:		January 2017
+Contact:	Chanwoo Choi <[e-mail]>
+Description:
+		The /sys/class/devfreq-event/event(x)/name attribute contains
+		the name of the devfreq-event object. This attribute is
+		read-only.
+
+What:		/sys/class/devfreq-event/event(x)/enable_count
+Date:		January 2017
+Contact:	Chanwoo Choi <[e-mail]>
+Description:
+		The /sys/class/devfreq-event/event(x)/enable_count attribute
+		contains the reference count to enable the devfreq-event
+		object. If the device is enabled, the value of attribute is
+		greater than zero.

drivers/devfreq/devfreq-event.c

···
 						struct devfreq_event_desc *desc)
 {
 	struct devfreq_event_dev *edev;
-	static atomic_t event_no = ATOMIC_INIT(0);
+	static atomic_t event_no = ATOMIC_INIT(-1);
 	int ret;
 
 	if (!dev || !desc)
···
 	edev->dev.class = devfreq_event_class;
 	edev->dev.release = devfreq_event_release_edev;
 
-	dev_set_name(&edev->dev, "event.%d", atomic_inc_return(&event_no) - 1);
+	dev_set_name(&edev->dev, "event%d", atomic_inc_return(&event_no));
 	ret = device_register(&edev->dev);
 	if (ret < 0) {
 		put_device(&edev->dev);