net: phy: mxl-gpy: add basic LED support

Add basic support for LEDs connected to MaxLinear GPY2xx and GPY115 PHYs.
The PHYs allow up to 4 LEDs to be connected.
Implement controlling LEDs in software as well as netdev trigger offloading
and LED polarity setup.

The hardware claims to support 16 PWM brightness levels but there is no
documentation on how to use that feature, hence this is not supported.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/b6ec9050339f8244ff898898a1cecc33b13a48fc.1727741563.git.daniel@makrotopia.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>