platform/x86: dell-*: Add a generic dell-laptop notifier chain

There are several cases where events handled in one of the dell-* drivers
need to be propagated to another dell-* driver.

This commit adds 3 generic functions:
dell_laptop_register_notifier()
dell_laptop_unregister_notifier()
dell_laptop_call_notifier()

It currently only defines 1 action:
DELL_LAPTOP_KBD_BACKLIGHT_BRIGHTNESS_CHANGED

Which is intended to propagate kbd_backlight_brightness_changed wmi
events from dell-wmi to dell-laptop (which contains the actual kbd
backlight driver).

These functions are put in dell-smbios as both dell-wmi and dell-laptop
use smbios functions and I do not want to put the notifier head in
either driver, as that will make the 2 drivers depend on each other.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>