Add support to defer core initialization for the endpoint mode of
operation.

This would enable support for implementations where the core
initialization needs to be deferred until the PCIe reference clock is
available from the host system.
Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
[lorenzo.pieralisi@arm.com: commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>