Device-Tree bindings for drivers/gpio/gpio-imx-rpmsg.c gpio driver over
rpmsg. On i.mx7ULP PTA PTB are connected on M4 side, so rpmsg gpio driver
needed to get/set gpio status from M4 side by rpmsg.

Required properties:
- compatible : Should be "fsl,imx-rpmsg-gpio".
- port_idx : Specify the GPIO PORT index, PTA:0, PTB:1.
- gpio-controller : Mark the device node as a gpio controller.
- #gpio-cells : Should be two. The first cell is the pin number and
  the second cell is used to specify the gpio polarity:
      0 = active high
      1 = active low

Note: Each GPIO port should have an alias correctly numbered in "aliases"
node.

Examples:

aliases {
	gpio4 = &rpmsg_gpio0;
	gpio5 = &rpmsg_gpio1;
};

rpmsg_gpio0: rpmsg-gpio0 {
	compatible = "fsl,imx-rpmsg-gpio";
	port_idx = <0>;
	gpio-controller;
	#gpio-cells = <2>;
	status = "okay";
};

rpmsg_gpio1: rpmsg-gpio1 {
	compatible = "fsl,imx-rpmsg-gpio";
	port_idx = <1>;
	gpio-controller;
	#gpio-cells = <2>;
	status = "okay";
};