rfkill: add notifier chains support

Add a notifier chain for use by the rfkill class.  This notifier chain
signals the following events (more to be added when needed):

  1. rfkill: rfkill device state has changed

A pointer to the rfkill struct will be passed as a parameter.

The notifier message types have been added to include/linux/rfkill.h
instead of to include/linux/notifier.h in order to avoid the madness of
modifying a header used globally (and that triggers an almost full tree
rebuild every time it is touched) with information that is of interest only
to code that includes the rfkill.h header.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>