dt-bindings: net: mdio: Make descriptions more general

A number of descriptions assume a PHY device, but since this binding
describes a MDIO bus which can have different kinds of MDIO devices
attached to it, rephrase some descriptions to be more general in that
regard.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>