dt/bindings: Add binding for the Raspberry Pi firmware driver

This driver will provide support for calls into the firmware that will
be used by other drivers like cpufreq and vc4.
Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/arm/bcm/raspberrypi,bcm2835-firmware.txt

+Raspberry Pi VideoCore firmware driver
+
+Required properties:
+
+- compatible:		Should be "raspberrypi,bcm2835-firmware"
+- mboxes:		Phandle to the firmware device's Mailbox.
+			  (See: ../mailbox/mailbox.txt for more information)
+
+Example:
+
+firmware {
+	compatible = "raspberrypi,bcm2835-firmware";
+	mboxes = <&mailbox>;
+};