dt: mailbox: add generic bindings

Define generic bindings for the framework clients to
request mailbox channels.

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

diff --git a/Documentation/devicetree/bindings/mailbox/mailbox.txt b/Documentation/devicetree/bindings/mailbox/mailbox.txt
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+* 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:
+- mboxes: List of phandle and mailbox channel specifiers.
+
+Optional property:
+- mbox-names: List of identifier strings for each mailbox channel
+               required by the client. The use of this property
+               is discouraged in favor of using index in list of
+               'mboxes' while requesting a mailbox. Instead the
+               platforms may define channel indices, in DT headers,
+               to something legible.
+
+Example:
+       pwr_cntrl: power {
+               ...
+               mbox-names = "pwr-ctrl", "rpc";
+               mboxes = <&mailbox 0
+                       &mailbox 1>;
+       };