dt-bindings: net: pcs: mediatek,sgmiisys: deprecate "mediatek,pnswap"

dt-bindings: net: pcs: mediatek,sgmiisys: deprecate "mediatek,pnswap"

Reference the common PHY properties, and update the example to use them.
Note that a PCS subnode exists, and it seems a better container of the
polarity description than the SGMIISYS node that hosts "mediatek,pnswap".
So use that.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20260119091220.1493761-4-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>