arm64: dts: Fix the base address of the Broadcom iProc mdio mux

Modify the base address of the mdio mux driver to point to the
start of the mdio mux 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>
Signed-off-by: David S. Miller <davem@davemloft.net>