net: pcs: lynx: add lynx_pcs_create_mdiodev()

Add lynx_pcs_create_mdiodev() to simplify the creation of the mdio
device associated with lynx PCS. In order to allow lynx_pcs_destroy()
to clean this up, we need to arrange for lynx_pcs_create() to take a
refcount on the mdiodev, and lynx_pcs_destroy() to put it.

Adding the refcounting to lynx_pcs_create()..lynx_pcs_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>
Reviewed-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Tested-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>