dt-bindings: firmware: Document cznic,turris-mox-rwtm binding

This adds device tree binding documentation for the driver communicating
with the rWTM firmware on Turris Mox.

Link: https://lore.kernel.org/r/20190822014318.19478-2-marek.behun@nic.czSigned-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/devicetree/bindings/firmware/cznic,turris-mox-rwtm.txt

+Turris Mox rWTM firmware driver
+
+Required properties:
+ - compatible		: Should be "cznic,turris-mox-rwtm"
+ - mboxes		: Must contain a reference to associated mailbox
+
+This device tree node should be used on Turris Mox, or potentially another A3700
+compatible device running the Mox's rWTM firmware in the secure processor (for
+example it is possible to flash this firmware into EspressoBin).
+
+Example:
+
+	firmware {
+		turris-mox-rwtm {
+			compatible = "cznic,turris-mox-rwtm";
+			mboxes = <&rwtm 0>;
+			status = "okay";
+		};
+	};

MAINTAINERS

@@ -1633,6 +1633,7 @@ S:	Maintained
 F:	Documentation/ABI/testing/debugfs-moxtet
 F:	Documentation/ABI/testing/sysfs-bus-moxtet-devices
 F:	Documentation/devicetree/bindings/bus/moxtet.txt
+F:	Documentation/devicetree/bindings/firmware/cznic,turris-mox-rwtm.txt
 F:	Documentation/devicetree/bindings/gpio/gpio-moxtet.txt
 F:	include/linux/moxtet.h
 F:	drivers/bus/moxtet.c