The PCI reset GPIO is active low, so represent it with the
GPIO_ACTIVE_LOW flag.

Even though the imx6 PCI driver will not take the polarity into account
in this case, it is better to provide a correct description in device-tree.
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>