HID: lg-g15: Add keyboard and LCD backlight control

Register a led_classdev for the keyboard backlight and another one for
the LCD backlight.

This commit also includes LED_BRIGHT_HW_CHANGED support, which together
with a desktop-environment which supports this, such as GNOME3 leads to
the kbd-backlight OSD being show with the new level when changing the
backlight setting through the hotkey on the keyboard.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>