net: phy: mdio-mvebu: Make the clock property required
This property is wrongly documented as 'optional', although all the
ethernet units need a clock to be enabled in order to access the MDIO
interface. For that reason, this commit makes the property a required
one and fails to probe the device, when a clock is not properly provided.
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>