The MediaTek MT7981 SoC comes with two 1G/2.5G SGMII ports, just like
MT7986.

In addition MT7981 is equipped with a built-in 1000Base-T PHY which can
be used with GMAC1.

As many MT7981 boards make use of inverting SGMII signal polarity, add
new device-tree attribute 'mediatek,pn_swap' to support them.
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>