cfg802154: add PM hooks

This patch help to implement suspend/resume in mac802154, these
hooks will be run before the device is suspended and after it
resumes.

Signed-off-by: Varka Bhadram <varkab@cdac.in>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>