dt-bindings: mailbox: Add Amlogic Meson MHU Bindings

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/meson-mhu.txt

+Amlogic Meson MHU Mailbox Driver
+================================
+
+The Amlogic's Meson SoCs Message-Handling-Unit (MHU) is a mailbox controller
+that has 3 independent channels/links to communicate with remote processor(s).
+MHU links are hardwired on a platform. A link raises interrupt for any
+received data. However, there is no specified way of knowing if the sent
+data has been read by the remote. This driver assumes the sender polls
+STAT register and the remote clears it after having read the data.
+
+Mailbox Device Node:
+====================
+
+Required properties:
+--------------------
+- compatible:		Shall be "amlogic,meson-gxbb-mhu"
+- reg:			Contains the mailbox register address range (base
+			address and length)
+- #mbox-cells		Shall be 1 - the index of the channel needed.
+- interrupts:		Contains the interrupt information corresponding to
+			each of the 2 links of MHU.
+
+Example:
+--------
+
+	mailbox: mailbox@c883c404 {
+		#mbox-cells = <1>;
+		compatible = "amlogic,meson-gxbb-mhu";
+		reg = <0 0xc883c404 0 0x4c>;
+		interrupts = <0 208 IRQ_TYPE_EDGE_RISING>,
+			     <0 209 IRQ_TYPE_EDGE_RISING>,
+			     <0 210 IRQ_TYPE_EDGE_RISING>;
+		#mbox-cells = <1>;
+	};