i.MX RPMSG platform implementations

Required properties:
- compatible : "fsl,imx7d-rpmsg", "fsl,imx6sx-rpmsg".
  "fsl,rpmsg-bus", "simple-bus", "fsl,imx8qxp-rpmsg".
  "fsl,imx8qm-rpmsg".
- vdev-nums : The number of the remote virtual devices.
- reg : The reserved DDR phisical memory used to store
  vring descriptors.
- multi-core-id: The id number of the remote processors.
  And it is optional for the legacy platforms, since they
  only have one remote processors.


=====================================================================
message unit module for RPMSG

- mu_rpmsg : The message unit module used to do the communications
  between the asymmetric cores.
- compatible : "fsl,imx6sx-mu", "fsl,imx-mu-rpmsg1".
  Different mu module would be used by the different remote processor.
  The "fsl, imx6sx-mu" is used by the first remote processor.
  The "fsl,imx-mu-rpmsg1" is used by the second remote process.
- reg : Should contain MU registers location and length.
- interrupts : interrupt mapping for RPMSG MU IRQ
- interrupt-parent : A single value that points to the interrupt
  parent to which the child domain is being mapped.
  Value must be "&intmux_cm40" or "&intmux_cm41"

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

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

imx_rpmsg: imx_rpmsg {
	compatible = "fsl,rpmsg-bus", "simple-bus";
	#address-cells = <2>;
	#size-cells = <2>;
	ranges;

	mu_rpmsg: mu_rpmsg@37440000 {
		compatible = "fsl,imx6sx-mu";
		reg = <0x0 0x37440000 0x0 0x10000>;
		interrupts = <31 IRQ_TYPE_LEVEL_HIGH>;
		interrupt-parent = <&intmux_cm40>;
		status = "okay";
	};

	rpmsg: rpmsg{
		compatible = "fsl,imx8qxp-rpmsg";
		status = "disabled";
	};
};