net: phy: add dependency on HAS_IOMEM to MDIO_BUS_MUX_MMIOREG

On UML builds, mdio-mux-mmioreg.c fails to compile:

drivers/net/phy/mdio-mux-mmioreg.c:50:3: error: implicit declaration of function ‘ioremap’ [-Werror=implicit-function-declaration]
drivers/net/phy/mdio-mux-mmioreg.c:63:3: error: implicit declaration of function ‘iounmap’ [-Werror=implicit-function-declaration]

This is due to CONFIG_OF now being user selectable. Add a dependency on
HAS_IOMEM to fix this.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>