ARM: dts: imx6qdl-sabresd: Pass the correct PCI reset polarity

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>