powerpc/mpc85xx_mds: Select PHYLIB only if NETDEVICES is enabled

PHYLIB depends on NETDEVICES, so to avoid unmet dependencies warning
from Kconfig it needs to be selected conditionally.

Also add checks if PHYLIB is built-in to avoid undefined references to
PHYLIB's symbols.

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