dpll: Add notifier chain for dpll events

dpll: Add notifier chain for dpll events

Currently, the DPLL subsystem reports events (creation, deletion, changes)
to userspace via Netlink. However, there is no mechanism for other kernel
components to be notified of these events directly.

Add a raw notifier chain to the DPLL core protected by dpll_lock. This
allows other kernel subsystems or drivers to register callbacks and
receive notifications when DPLL devices or pins are created, deleted,
or modified.

Define the following:
- Registration helpers: {,un}register_dpll_notifier()
- Event types: DPLL_DEVICE_CREATED, DPLL_PIN_CREATED, etc.
- Context structures: dpll_{device,pin}_notifier_info  to pass relevant
  data to the listeners.

The notification chain is invoked alongside the existing Netlink event
generation to ensure in-kernel listeners are kept in sync with the
subsystem state.

Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Co-developed-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: Petr Oros <poros@redhat.com>
Reviewed-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Link: https://patch.msgid.link/20260203174002.705176-4-ivecera@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>