net: pcs: lynx: add support for 25GBASE-R

The Lynx PCS never supported 25GBASE-R. Add it, so that a 25G DPMAC on
LX2160A can be driven by phylink.

The SerDes side had the mode since the commit
  1cab8fba5073 ("phy: lynx-28g: add support for 25GBASER")
and dpaa2-mac maps DPMAC_ETH_IF_CAUI to PHY_INTERFACE_MODE_25GBASER,
but the PCS never handled it, so phylink_validate() failed with the
interface.

25GBASE-R is a serial mode without clause-37 in-band autonegotiation,
exactly like 10GBASE-R: it does not needs any configuration, its state
is read with phylink_mii_c45_pcs_get_state(), and it reports
LINK_INBAND_DISABLE.

phylink_mii_c45_pcs_get_state() is extended in the same patch because
without it the link comes up reporting Unknown/Unknown.

Suggested-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Co-developed-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Vincent Jardin <vjardin@free.fr>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20260922-for-upstream-lynx-25gbaser-v1-v2-1-1e6cf79c7b52@free.fr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>