r6040: implement phylib

This patch adds support for using phylib and adds the required mdiobus driver
stubs. This allows for less code to be present in the driver and removes
the PHY status specific timer which is now handled by phylib directly.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>