dt-bindings: mfd: Add A33 GPADC binding

This patch adds documentation for the A33 GPADC binding.
Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/sun4i-gpadc.txt

+Allwinner SoCs' GPADC Device Tree bindings
+------------------------------------------
+The Allwinner SoCs all have an ADC that can also act as a thermal sensor
+and sometimes as a touchscreen controller.
+
+Required properties:
+  - compatible: "allwinner,sun8i-a33-ths",
+  - reg: mmio address range of the chip,
+  - #thermal-sensor-cells: shall be 0,
+  - #io-channel-cells: shall be 0,
+
+Example:
+	ths: ths@01c25000 {
+		compatible = "allwinner,sun8i-a33-ths";
+		reg = <0x01c25000 0x100>;
+		#thermal-sensor-cells = <0>;
+		#io-channel-cells = <0>;
+	};
+
+sun4i, sun5i and sun6i SoCs are also supported via the older binding:
+
+sun4i resistive touchscreen controller
+--------------------------------------
+
+Required properties:
+ - compatible: "allwinner,sun4i-a10-ts", "allwinner,sun5i-a13-ts" or
+   "allwinner,sun6i-a31-ts"
+ - reg: mmio address range of the chip
+ - interrupts: interrupt to which the chip is connected
+ - #thermal-sensor-cells: shall be 0
+
+Optional properties:
+ - allwinner,ts-attached	 : boolean indicating that an actual touchscreen
+				   is attached to the controller
+ - allwinner,tp-sensitive-adjust : integer (4 bits)
+				   adjust sensitivity of pen down detection
+				   between 0 (least sensitive) and 15
+				   (defaults to 15)
+ - allwinner,filter-type	 : integer (2 bits)
+				   select median and averaging filter
+				   samples used for median / averaging filter
+				   0: 4/2
+				   1: 5/3
+				   2: 8/4
+				   3: 16/8
+				   (defaults to 1)
+
+Example:
+
+	rtp: rtp@01c25000 {
+		compatible = "allwinner,sun4i-a10-ts";
+		reg = <0x01c25000 0x100>;
+		interrupts = <29>;
+		allwinner,ts-attached;
+		#thermal-sensor-cells = <0>;
+		/* sensitive/noisy touch panel */
+		allwinner,tp-sensitive-adjust = <0>;
+		allwinner,filter-type = <3>;
+	};