net: pcs: pcs-mtk-lynxi: deprecate "mediatek,pnswap"

net: pcs: pcs-mtk-lynxi: deprecate "mediatek,pnswap"

Prefer the new "rx-polarity" and "tx-polarity" properties, which in this
case have the advantage that polarity inversion can be specified per
direction (and per protocol, although this isn't useful here).

We use the vendor specific ones as fallback if the standard description
doesn't exist.

Daniel, referring to the Mediatek SDK, clarifies that the combined
SGMII_PN_SWAP_TX_RX register field should be split like this: bit 0 is
TX and bit 1 is RX:
https://lore.kernel.org/linux-phy/aSW--slbJWpXK0nv@makrotopia.org/

Suggested-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20260119091220.1493761-6-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>