Blackfin EMAC driver: Add phy abstraction layer supporting in bfin_emac driver

 - add MDIO functions and register mdio bus
 - add phy abstraction layer (PAL) functions and use PAL API
 - test on STAMP537 board

Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Acked-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>