dt-bindings: net: ethernet-phy: clarify when compatible must specify PHY ID

dt-bindings: net: ethernet-phy: clarify when compatible must specify PHY ID

Change PHY ID description in ethernet-phy.yaml to clarify that a
PHY ID is required (may -> must) when the PHY requires special
initialization sequence.

Link: https://lore.kernel.org/netdev/20251026212026.GA2959311-robh@kernel.org/
Link: https://lore.kernel.org/netdev/aQIZvDt5gooZSTcp@debianbuilder/
Signed-off-by: Buday Csaba <buday.csaba@prolan.hu>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/64c52d1a726944a68a308355433e8ef0f82c4240.1762157515.git.buday.csaba@prolan.hu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>