dt-bindings: mfd: Add retu/tahvo ASIC chips bindings

There are Device Tree source files defining a device node for the
retu/tahvo I2C chip, but there isn't a DT binding document for it.
Signed-off-by: NJavier Martinez Canillas <javier@dowhile0.org>
Acked-by: NRob Herring <robh@kernel.org>
Acked-by: NAaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: NTony Lindgren <tony@atomide.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/retu.txt

+* Device tree bindings for Nokia Retu and Tahvo multi-function device
+
+Retu and Tahvo are a multi-function devices found on Nokia Internet
+Tablets (770, N800 and N810). The Retu chip provides watchdog timer
+and power button control functionalities while Tahvo chip provides
+USB transceiver functionality.
+
+Required properties:
+- compatible:		"nokia,retu" or "nokia,tahvo"
+- reg:			Specifies the CBUS slave address of the ASIC chip
+- interrupts: 		The interrupt line the device is connected to
+- interrupt-parent:	The parent interrupt controller
+
+Example:
+
+cbus0 {
+	compatible = "i2c-cbus-gpio";
+	...
+	retu: retu@1 {
+		compatible = "nokia,retu";
+		interrupt-parent = <&gpio4>;
+		interrupts = <12 IRQ_TYPE_EDGE_RISING>;
+		reg = <0x1>;
+	};
+};