mdio: mux: Enhanced MDIO mux framework for integrated multiplexers

An integrated multiplexer uses same address space for
"muxed bus selection" and "generation of mdio transaction"
hence its good to register parent bus from mux driver.

Hence added a mechanism where mux driver could register a
parent bus and pass it down to framework via mdio_mux_init api.

Signed-off-by: Pramod Kumar <pramod.kumar@broadcom.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>