net: pcs: xpcs: add xpcs_create_mdiodev()

commit  db88caaa2b3e5cc33a3f0b9cf5fb309febacf167 from
https://github.com/nxp-imx/linux-imx lf-6.1.y

Add xpcs_create_mdiodev() to simplify the creation of the mdio device
associated with the XPCS. In order to allow xpcs_destroy() to clean
this up, we need to arrange for xpcs_create() to take a refcount on
the mdiodev, and xpcs_destroy() to put it.

Adding the refcounting to xpcs_create()..xpcs_destroy() will be
transparent to existing users of these interfaces.

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 9a5d500c

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