dt-binding: add imx rpmsg compatibility documentation

Add imx rpmsg compatibility documentation.

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>

Documentation/devicetree/bindings/soc/fsl/fsl,rpmsg.txt

@@ -0,0 +1,75 @@
+i.MX RPMSG platform implementations
+
+Distributed framework is used in IMX RPMSG implementation, refer to the
+following requirements:
+  - The CAN functions contained in M core and RTOS should be ready and
+    complete functional in 50ms after AMP system is turned on.
+  - Partition reset. System wouldn't be stalled by the exceptions (e.x
+    the reset triggered by the system hang) occurred at the other side.
+    And the RPMSG mechanism should be recovered automactilly after the
+    partition reset is completed.
+In this scenario, the M core and RTOS would be kicked off by bootloader
+firstly, then A core and Linux would be loaded later. Both M core/RTOS
+and A core/Linux are running independly.
+One physical memory region used to store the vring is mandatory required
+to pre-reserved and well-knowned by both A core and M core
+
+Required properties:
+- compatible: "fsl,imx8qxp-rpmsg", "fsl,imx8mq-rpmsg".
+- vdev-nums: The number of the remote virtual devices.
+- reg: The reserved phisical DDR memory used to store vring descriptors.
+
+Optional properties:
+- rpmsg_dma_reserved: The reserved per device dma pool, that used to
+  allocate the shared memory buffers from the per device.
+  And it is optional for some platforms, since the system dma pool
+  is used to allocate the shared memory buffers directly on them.
+- mub-partition: The partition ID of muB side, that's optional
+  and used on i.mx8qm/8qxp for partition reset. The default
+  value is 3 in driver without this property.
+
+=====================================================================
+Mailbox used by iMX RPMSG
+
+- mboxes: mailboxes used in the RPMSG transactions.
+- mbox-names: names of the mailboxes used in RPMSG.
+	- "tx":TX channel with 32bit transmit register and IRQ transmit
+	- "rx":RX channel with 32bit receive register and IRQ support
+	- "rxdb":RX doorbell channel.
+
+Example:
+Rpmsg node in board dts file.
+&rpmsg{
+	/*
+	 * 64K for one rpmsg instance:
+	 */
+	vdev-nums = <2>;
+	reg = <0x0 0x90000000 0x0 0x20000>;
+	status = "okay";
+};
+
+SOC level dts node definitions:
+rpmsg_reserved: rpmsg@0x90000000 {
+	no-map;
+	reg = <0 0x90000000 0 0x400000>;
+};
+rpmsg_dma_reserved:rpmsg_dma@0x90400000 {
+	compatible = "shared-dma-pool";
+	no-map;
+	reg = <0 0x90400000 0 0x1C00000>;
+};
+rpmsg: rpmsg{
+	compatible = "fsl,imx8qxp-rpmsg";
+	/* up to now, the following channels are used in imx rpmsg
+	 * - tx1/rx1: messages channel.
+	 * - general interrupt1: remote proc finish re-init rpmsg stack
+	 *   when A core is partition reset.
+	 */
+	mbox-names = "tx", "rx", "rxdb";
+	mboxes = <&lsio_mu5 0 1
+		  &lsio_mu5 1 1
+		  &lsio_mu5 3 1>;
+	mub-partition = <3>;
+	memory-region = <&rpmsg_dma_reserved>;
+	status = "disabled";
+};