net: phy: mdio: add missing __iomem annotation

Added missing __iomem annotation in order to fix the following
sparse warnings:

drivers/net/phy/mdio-mux-mmioreg.c:51:27: warning: incorrect type in initializer (different address spaces)
drivers/net/phy/mdio-mux-mmioreg.c:51:27:    expected void *p
drivers/net/phy/mdio-mux-mmioreg.c:51:27:    got void [noderef] <asn:2>*
drivers/net/phy/mdio-mux-mmioreg.c:57:21: warning: incorrect type in argument 1 (different address spaces)
drivers/net/phy/mdio-mux-mmioreg.c:57:21:    expected void const volatile [noderef] <asn:2>*addr
drivers/net/phy/mdio-mux-mmioreg.c:57:21:    got void *p
drivers/net/phy/mdio-mux-mmioreg.c:60:25: warning: incorrect type in argument 2 (different address spaces)
drivers/net/phy/mdio-mux-mmioreg.c:60:25:    expected void volatile [noderef] <asn:2>*addr
drivers/net/phy/mdio-mux-mmioreg.c:60:25:    got void *p
drivers/net/phy/mdio-mux-mmioreg.c:64:25: warning: incorrect type in argument 1 (different address spaces)
drivers/net/phy/mdio-mux-mmioreg.c:64:25:    expected void volatile [noderef] <asn:2>*addr
drivers/net/phy/mdio-mux-mmioreg.c:64:25:    got void *p

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>