dt-bindings: mailbox: Document armada-3700-rwtm-mailbox binding

This adds device tree binding documentation for the rWTM BIU mailbox
driver on the Armada 37xx SOC (EspressoBin, Turris Mox).
Signed-off-by: Marek Behun <marek.behun@nic.cz>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/marvell,armada-3700-rwtm-mailbox.txt

+* rWTM BIU Mailbox driver for Armada 37xx
+
+Required properties:
+- compatible:	must be "marvell,armada-3700-rwtm-mailbox"
+- reg:		physical base address of the mailbox and length of memory mapped
+		region
+- interrupts:	the IRQ line for the mailbox
+- #mbox-cells:	must be 1
+
+Example:
+	rwtm: mailbox@b0000 {
+		compatible = "marvell,armada-3700-rwtm-mailbox";
+		reg = <0xb0000 0x100>;
+		interrupts = <GIC_SPI 18 IRQ_TYPE_LEVEL_HIGH>;
+		#mbox-cells = <1>;
+	};