net: cadence: macb: implement EEE TX LPI support

The GEM MAC has hardware LPI registers (NCR bit 19: TXLPIEN) but no
built-in idle timer, so asserting TXLPIEN blocks all TX immediately
with no automatic wake. A software idle timer is required, as noted
in Microchip documentation (section 40.6.19): "It is best to use
firmware to control LPI."

Implement phylink managed EEE using the mac_enable_tx_lpi and
mac_disable_tx_lpi callbacks:

- macb_tx_lpi_set(): sets or clears TXLPIEN; requires bp->lock to be
  held by the caller (asserted with lockdep_assert_held). Returns bool
  indicating whether the register actually changed, avoiding redundant
  writes and unnecessary udelay on the xmit fast path.

- macb_tx_lpi_work_fn(): delayed_work handler that enters LPI if all
  TX queues are idle and EEE is still active. Takes bp->lock with
  irqsave before calling macb_tx_lpi_set().

- macb_tx_lpi_schedule(): arms the work timer using the LPI timer
  value provided by phylink (default 250 ms). Called from
  macb_tx_complete() after each TX drain so the idle countdown
  restarts whenever the ring goes quiet.

- macb_tx_lpi_wake(): called from macb_start_xmit() under bp->lock,
  immediately before TSTART. Returns early if eee_active is false to
  avoid a register read on the common path when EEE is disabled.
  Clears TXLPIEN and applies a 50 us udelay for PHY wake (IEEE
  802.3az Tw_sys_tx is 16.5 us for 1000BASE-T / 30 us for
  100BASE-TX; GEM has no hardware enforcement). Only delays when
  TXLPIEN was actually set. The delay is placed after tx_head is
  advanced so the work_fn's queue-idle check sees a non-empty ring
  and cannot race back into LPI before the frame is transmitted.

- mac_enable_tx_lpi: stores the timer and sets eee_active under
  bp->lock, then defers the first LPI entry by 1 second per IEEE
  802.3az section 22.7a.

- mac_disable_tx_lpi: cancels the work (sync, without the lock to
  avoid deadlock with the work_fn), then takes bp->lock to clear
  eee_active and deassert TXLPIEN.

Populate phylink_config lpi_interfaces (MII, GMII, RGMII variants)
and lpi_capabilities (MAC_100FD | MAC_1000FD) so phylink can
negotiate EEE with the PHY and call the callbacks appropriately.
Set lpi_timer_default to 250000 us and eee_enabled_default to true.

Reviewed-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>
Reviewed-by: Théo Lebrun <theo.lebrun@bootlin.com>
Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
Link: https://patch.msgid.link/20260304105432.631186-3-nb@tipi-net.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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