PCI: endpoint: Use callback mechanism for passing events from EPC to EPF

Instead of using the notifiers for passing the events from EPC to EPF,
let's introduce a callback based mechanism where the EPF drivers can
populate relevant callbacks for EPC events they want to subscribe.

The use of notifiers in kernel is not recommended if there is a real link
between the sender and receiver, like in this case. Also, the existing
atomic notifier forces the notification functions to be in atomic context
while the caller may be in non-atomic context. For instance, the two
in-kernel users of the notifiers, pcie-qcom and pcie-tegra194, both are
calling the notifier functions in non-atomic context (from threaded IRQ
handlers). This creates a sleeping in atomic context issue with the
existing EPF_TEST driver that calls the EPC APIs that may sleep.

For all these reasons, let's get rid of the notifier chains and use the
simple callback mechanism for signalling the events from EPC to EPF
drivers. This preserves the context of the caller and avoids the latency
of going through a separate interface for triggering the notifications.

As a first step of the transition, the core_init() callback is introduced
in this commit, that'll replace the existing CORE_INIT notifier used for
signalling the init complete event from EPC.

During the occurrence of the event, EPC will go over the list of EPF
drivers attached to it and will call the core_init() callback if available.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Acked-by: Kishon Vijay Abraham I <kishon@kernel.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>