Mailbox: Generic: Specify mailbox api bindings

Due to the platform specific nature of remote's firmware, we can't do much
to facilitate shareable client drivers. So the Mailbox api is more like a
bunch of useful functions put together. The only feature that can be
abstracted out is mailbox channel assignment to client drivers, which this
binding specifies.

Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>

Documentation/devicetree/bindings/mailbox/mailbox.txt

@@ -0,0 +1,33 @@
+* Generic Mailbox Controller and client driver bindings
+
+Generic binding to provide a way for Mailbox controller drivers to
+assign appropriate mailbox channel to client drivers.
+
+* Mailbox Controller
+
+Required property:
+- #mbox-cells: Must be at least 1. Number of cells in a mailbox
+		specifier.
+
+Example:
+	mailbox: mailbox {
+		...
+		#mbox-cells = <1>;
+	};
+
+
+* Mailbox Client
+
+Required property:
+- mbox: List of phandle and mailbox channel specifier.
+
+- mbox-names: List of identifier strings for each mailbox channel
+		required by the client.
+
+Example:
+	pwr_cntrl: power {
+		...
+		mbox-names = "pwr-ctrl", "rpc";
+		mbox = <&mailbox 0
+			&mailbox 1>;
+	};