Blackfin EMAC driver: add a select for the PHYLIB of this driver

Since we are adding requirement for the PHYLIB for this driver, there should be a select for that

Cc: Robin Getz <robin.getz@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>