net: phy: mediatek-ge-soc: support PHY LEDs

Implement netdev trigger and primitive bliking offloading as well as
simple set_brigthness function for both PHY LEDs of the in-SoC PHYs
found in MT7981 and MT7988.

For MT7988, read boottrap register and apply LED polarities accordingly
to get uniform behavior from all LEDs on MT7988.
This requires syscon phandle 'mediatek,pio' present in parenting MDIO bus
which should point to the syscon holding the boottrap register.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/dc324d48c00cd7350f3a506eaa785324cae97372.1691977904.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>