ARM: dts: imx: Fix Ethernet PHY reset polarity

The FEC driver ignores the GPIO polarity from 'phy-reset-gpios' and
considers that the Ethernet PHY is active low, unless the
property 'phy-reset-active-high' is present.

Fix the device tree description by explicitly passing the
'GPIO_ACTIVE_LOW' flag to the 'phy-reset-gpios' property.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>