ARM: dts: imx6: Fix PCI GPIO reset polarity

The imx6 PCI driver ignores the GPIO polarity from 'reset-gpio' and
considers that the PCI reset is active low, unless the
property 'reset-gpio-active-high' is present.

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

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>