net: mdio: add mdio_device_get() and mdio_device_put()

Add two new operations for a mdio device to manage the refcount on the
underlying struct device. This will be used by mdio PCS drivers to
simplify the creation and destruction handling, making it easier for
users to get it correct.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>