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: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>