We should not assume that the PHY reset is always active low.

Retrieve this information from the device tree instead, so that the PHY reset
can work on both cases.
Reported-by: NPhilipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: NFabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>