ice: implement E825 TX ref clock control and TXC hardware sync status

Build on the previously introduced TXC DPLL framework and implement
full TX reference clock control and hardware-backed synchronization
status reporting for E825 devices.

E825 firmware may accept or override TX reference clock requests based
on device-wide routing constraints and link conditions. Because the
final selection becomes visible only after a link-up event, the driver
splits the observation into two complementary signals:

  - TXCLK pin state reflects the requested TX reference clock
    (pf->ptp.port.tx_clk_req). After a link-up, the value is reconciled
    against the SERDES reference selector by
    ice_txclk_update_and_notify(); if firmware or auto-negotiation
    selected a different clock, tx_clk_req is overwritten so that pin
    state converges to the actual hardware selection.

  - TXC DPLL lock status reflects hardware synchronization:
      * LOCKED   when an external TX reference is in use
      * UNLOCKED when falling back to ENET/TXCO, or when a requested
        external reference has not (yet) been accepted by hardware.

Userspace observing only pin state therefore sees user intent, while
lock status is the authoritative indicator of whether the requested
clock is actually selected and synchronizing. This matches the DPLL
subsystem model where pin state describes topology and device lock
status describes signal quality.

TX reference selection topology:
  - External references (SYNCE, EREF0) are represented as TXCLK pins
  - The internal ENET/TXCO clock has no pin representation; when
    selected, all TXCLK pins are reported DISCONNECTED

With this change, TX reference clocks on E825 devices can be reliably
selected, observed via standard DPLL interfaces, and monitored for
effective synchronization through TXC DPLL lock status.

Reviewed-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Signed-off-by: Grzegorz Nitka <grzegorz.nitka@intel.com>
Link: https://patch.msgid.link/20260607183045.1213735-14-grzegorz.nitka@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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