i.MX RPMSG platform implementations

Required properties:
- compatible : "fsl,imx7d-rpmsg", "fsl,imx6sx-rpmsg"
- vdev-nums : The number of the remote virtual devices.
- reg : The reserved DDR phisical memory used to store
  vring descriptors.

Example:
rpmsg: rpmsg{
	compatible = "fsl,imx6sx-rpmsg";
	status = "disabled";
};

&rpmsg{
	vdev-nums = <1>;
	reg = <0xbfff0000 0x10000>;
	status = "okay";
};
