PCI: endpoint: Use link_up() callback in place of LINK_UP notifier

As a part of the transition towards callback mechanism for signalling the
events from EPC to EPF, let's use the link_up() callback in the place of
the LINK_UP notifier. This also removes the notifier support completely
from the PCI endpoint framework.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>