powerpc/e8248e: Select PHYLIB only if NETDEVICES is enabled

Select PHYLIB only if NETDEVICES is enabled and MDIO_BITBANG only if
PHYLIB is present to avoid warnings from Kconfig.

To prevent undefined references during linking register MDIO driver only
if CONFIG_MDIO_BITBANG is enabled.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Scott Wood <oss@buserror.net>