power_domain: Fix notification initialization

Within process_power_state_transition_notification_response
a notification is sent but it may carry unknown values.
This is a fix to ensure that the notification about to be sent
is zero-initialized.

Change-Id: If8e6a19c214b3461587a65d8e3bc8f36273ce577
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>