ethernet: smsc91x: select MDIO

This driver implements an MDIO device. Enable the MDIO driver to avoid a
build error for:

orphan section `._device.static.3_CONFIG_MDIO_INIT_PRIORITY_'

Also set the proper compatible in mdio_shell so that the ethernet tests
build correctly.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>