Introduce event_notify variant function to allow vendor to get notification
of important events and connect to any proprietary debugging facilities.

Link: https://lore.kernel.org/r/20201205115901.26815-4-stanley.chu@mediatek.comReviewed-by: Asutosh Das <asutoshd@codeaurora.org>
Reviewed-by: Can Guo <cang@codeaurora.org>
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>