dt/bindings: Add binding for the BCM2835 mailbox driver

This patch was split out of Lubomir's original mailbox patch by Eric
Anholt, and the required properties documentation and examples have
been filled out more completely and updated for the driver being
changed to expose a single channel.
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Craig McGeachie <slapdau@yahoo.com.au>
Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/brcm,bcm2835-mbox.txt

+Broadcom BCM2835 VideoCore mailbox IPC
+
+Required properties:
+
+- compatible:	Should be "brcm,bcm2835-mbox"
+- reg:		Specifies base physical address and size of the registers
+- interrupts:	The interrupt number
+		  See bindings/interrupt-controller/brcm,bcm2835-armctrl-ic.txt
+- #mbox-cells:	Specifies the number of cells needed to encode a mailbox
+		  channel. The value shall be 0, since there is only one
+		  mailbox channel implemented by the device.
+
+Example:
+
+mailbox: mailbox@7e00b800 {
+	compatible = "brcm,bcm2835-mbox";
+	reg = <0x7e00b880 0x40>;
+	interrupts = <0 1>;
+	#mbox-cells = <0>;
+};
+
+firmware: firmware {
+	compatible = "raspberrypi,firmware";
+	mboxes = <&mailbox>;
+	#power-domain-cells = <1>;
+};