dt-bindings: net: Fix Broadcom iProc mdio mux driver base address

Modify the base address of the Broadcom iProc MDIO mux driver to
point to the start of the block's register address space.

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>