ARM: dts: imx53-cx9020: Fix the Ethernet PHY reset GPIO polarity

As explained in Documentation/devicetree/bindings/net/fsl-fec.txt the
phy-reset-gpios is active high only if the 'phy-reset-active-high' is
present.

As 'phy-reset-active-high' is not used here, fix the device tree
description by passing GPIO_ACTIVE_LOW flag.

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