PCI: dwc: Add API to notify core initialization completion

Add a new API dw_pcie_ep_init_notify() to let platform drivers
call it when the core is available 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>