net: phy: Introduce PHY ports representation

Ethernet provides a wide variety of layer 1 protocols and standards for
data transmission. The front-facing ports of an interface have their own
complexity and configurability.

Introduce a representation of these front-facing ports. The current code
is minimalistic and only support ports controlled by PHY devices, but
the plan is to extend that to SFP as well as raw Ethernet MACs that
don't use PHY devices.

This minimal port representation allows describing the media and number
of pairs of a BaseT port. From that information, we can derive the
linkmodes usable on the port, which can be used to limit the
capabilities of an interface.

For now, the port pairs and medium is derived from devicetree, defined
by the PHY driver, or populated with default values (as we assume that
all PHYs expose at least one port).

The typical example is 100M ethernet. 100BaseTX works using only 2
pairs on a Cat 5 cables. However, in the situation where a 10/100/1000
capable PHY is wired to its RJ45 port through 2 pairs only, we have no
way of detecting that. The "max-speed" DT property can be used, but a
more accurate representation can be used :

mdi {
	connector-0 {
		media = "BaseT";
		pairs = <2>;
	};
};

From that information, we can derive the max speed reachable on the
port.

Another benefit of having that is to avoid vendor-specific DT properties
(micrel,fiber-mode or ti,fiber-mode).

This basic representation is meant to be expanded, by the introduction
of port ops, userspace listing of ports, and support for multi-port
devices.

Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20260108080041.553250-4-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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