dt-bindings: net: Add clock handle to Broadcom iProc mdio mux

Add clock phandle, of the core clock driving the mdio block, as an
optional property to the Broadcom iProc mdio mux.

The clock, when specified, will be used to setup the rate adjust registers
in the mdio to derrive the mdio's operating frequency.

Signed-off-by: Arun Parameswaran <arun.parameswaran@broadcom.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>