notifier: Pass data argument to callback

notifier: Pass data argument to callback

This allows to pass additional information to the notifier callback
which is useful if sender and receiver do not share any other distinct
data structure.

Will be used first for the clock reset notifier.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>