PCI: endpoint: Add core init notifying feature

Add a new feature core_init_notifier for cores that can notify about
their availability for initialization.

Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>