drivers: net: sun4i-emac: select MDIO_SUN4I

The EMAC driver can't work without its associated PHY driver. Reflect
this in the Kconfig options by selecting it.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>