dt-bindings: net: mdio: Document common properties

Some of the properties pertaining to the broken turn around or resets
were only documented in ethernet-phy.yaml while they are applicable
across all MDIO devices and not Ethernet PHYs specifically which are a
superset.

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