ARM: dts: imx6-sabreauto: add reset-gpios property for max7310_b

The reset pin to the port expander chip (MAX7310) is controlled by a gpio,
so define a reset-gpios property to control it. There are three MAX7310's
on the SabreAuto CPU card (max7310_[abc]), but all use the same pin for
their reset. Since all can't acquire the same pin, assign it to max7310_b,
that chip is needed by more functions (usb and adv7180).

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