leds: class: Add new optional brightness_hw_changed attribute

Some LEDs may have their brightness level changed autonomously
(outside of kernel control) by hardware / firmware. This commit
adds support for an optional brightness_hw_changed attribute to
signal such changes to userspace (if a driver can detect them):

What:		/sys/class/leds/<led>/brightness_hw_changed
Date:		January 2017
KernelVersion:	4.11
Description:
		Last hardware set brightness level for this LED. Some LEDs
		may be changed autonomously by hardware/firmware. Only LEDs
		where this happens and the driver can detect this, will
		have this file.

		This file supports poll() to detect when the hardware
		changes the brightness.

		Reading this file will return the last brightness level set
		by the hardware, this may be different from the current
		brightness.

Drivers which want to support this, simply add LED_BRIGHT_HW_CHANGED to
their flags field and call led_classdev_notify_brightness_hw_changed()
with the hardware set brightness when they detect a hardware / firmware
triggered brightness change.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>