dt-bindings: mfd: Add ST Multi-Function eXpander (STMFX) core bindings

This patch adds documentation of device tree bindings for the
STMicroelectronics Multi-Function eXpander (STMFX) MFD core.
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/stmfx.txt

+STMicroelectonics Multi-Function eXpander (STMFX) Core bindings
+
+ST Multi-Function eXpander (STMFX) is a slave controller using I2C for
+communication with the main MCU. Its main features are GPIO expansion, main
+MCU IDD measurement (IDD is the amount of current that flows through VDD) and
+resistive touchscreen controller.
+
+Required properties:
+- compatible: should be "st,stmfx-0300".
+- reg: I2C slave address of the device.
+- interrupts: interrupt specifier triggered by MFX_IRQ_OUT signal.
+  Please refer to ../interrupt-controller/interrupt.txt
+
+Optional properties:
+- drive-open-drain: configure MFX_IRQ_OUT as open drain.
+- vdd-supply: phandle of the regulator supplying STMFX.
+
+Example:
+
+	stmfx: stmfx@42 {
+		compatible = "st,stmfx-0300";
+		reg = <0x42>;
+		interrupts = <8 IRQ_TYPE_EDGE_RISING>;
+		interrupt-parent = <&gpioi>;
+		vdd-supply = <&v3v3>;
+	};
+
+Please refer to ../pinctrl/pinctrl-stmfx.txt for STMFX GPIO expander function bindings.