dt-bindings: mailbox: imx-mu: add generic MU channel support

Each MU has four pairs of rx/tx data register with four rx/tx interrupts
which can also be used as a separate channel.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/fsl,mu.txt

@@ -18,11 +18,31 @@ Messaging Unit Device Node:
 Required properties:
 -------------------
 - compatible :	should be "fsl,<chip>-mu", the supported chips include
-		imx8qxp, imx8qm.
+		imx6sx, imx7s, imx8qxp, imx8qm.
+		The "fsl,imx6sx-mu" compatible is seen as generic and should
+		be included together with SoC specific compatible.
 - reg :		Should contain the registers location and length
 - interrupts :	Interrupt number. The interrupt specifier format depends
 		on the interrupt controller parent.
-- #mbox-cells:  Must be 0. Number of cells in a mailbox
+- #mbox-cells:  Must be 2.
+			  <&phandle type channel>
+			    phandle   : Label name of controller
+			    type      : Channel type
+			    channel   : Channel number
+
+		This MU support 4 type of unidirectional channels, each type
+		has 4 channels. A total of 16 channels. Following types are
+		supported:
+		0 - TX channel with 32bit transmit register and IRQ transmit
+		acknowledgment support.
+		1 - RX channel with 32bit receive register and IRQ support
+		2 - TX doorbell channel. Without own register and no ACK support.
+		3 - RX doorbell channel.
+
+Optional properties:
+-------------------
+- clocks :	phandle to the input clock.
+- fsl,mu-side-b : Should be set for side B MU.
 
 Examples:
 --------
@@ -30,5 +50,5 @@ lsio_mu0: mailbox@5d1b0000 {
 	compatible = "fsl,imx8qxp-mu";
 	reg = <0x0 0x5d1b0000 0x0 0x10000>;
 	interrupts = <GIC_SPI 176 IRQ_TYPE_LEVEL_HIGH>;
-	#mbox-cells = <0>;
+	#mbox-cells = <2>;
 };