drivers: net: phy: mdio-mux: Add support for Generic Mux controls

Add support for Generic Mux controls, when Mdio mux node is a consumer
of mux produced by some other device.

Signed-off-by: Pankaj Bansal <pankaj.bansal@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>