ice: introduce TXC DPLL device and TX ref clock pin framework for E825

ice: introduce TXC DPLL device and TX ref clock pin framework for E825

E825 devices provide a dedicated TX clock (TXC) domain which may be
driven by multiple reference clock sources, including external board
references and port-derived SyncE. To support future TX clock control
and observability through the Linux DPLL subsystem, introduce a
separate TXC DPLL device (of DPLL_TYPE_GENERIC) and a framework for
representing TX reference clock inputs.

This change adds a new internal DPLL pin type (TXCLK) and registers
TX reference clock pins for E825-based devices:
- EXT_EREF0: a board-level external electrical reference
- SYNCE: a port-derived SyncE reference described via firmware nodes

The TXC DPLL device is created and managed alongside the existing
PPS and EEC DPLL instances. TXCLK pins are registered directly or
deferred via a notifier when backed by fwnode-described pins.
A per-pin attribute encodes the TX reference source associated with
each TXCLK pin.

At this stage, TXCLK pin state callbacks and TXC DPLL lock status
reporting are implemented as placeholders. Pin state getters always
return DISCONNECTED, and the TXC DPLL is initialized in the UNLOCKED
state. No hardware configuration or TX reference switching is
performed yet.

This patch establishes the structural groundwork required for
hardware-backed TX reference selection, verification, and
synchronization status reporting, which will be implemented in
subsequent patches.

Also signal dpll_init from the fwnode pin init error path so any
notifier worker already blocked on it can drain, avoiding a
flush_workqueue() deadlock during teardown.

Reviewed-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Grzegorz Nitka <grzegorz.nitka@intel.com>
Link: https://patch.msgid.link/20260607183045.1213735-11-grzegorz.nitka@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>