netdev/phy: mdio-mux-mmioreg.c should include of_address.h

mdio-mux-mmioreg.c uses function of_address_to_resource(), which is defined
in linux/of_address.h.  This fixes a compilation error:

drivers/net/phy/mdio-mux-mmioreg.c: In function 'mdio_mux_mmioreg_probe':
drivers/net/phy/mdio-mux-mmioreg.c:83:2: error: implicit declaration of
	function 'of_address_to_resource'

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>