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

commit  8230ca4cd7b61d6ea413e66f03e3be52d85766ad from
https://github.com/nxp-imx/linux-imx lf-6.1.y

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>
(cherry picked from commit c4933fa8

)
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>