Documentation: Detail permitted DT properties for the BU21013 Touch Screen

Here we apply required documentation for the Rohm BU21013 Touch
Screen driver which describe available properties and how to use
them.

Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: linux-input@vger.kernel.org
Cc: devicetree-discuss@lists.ozlabs.org
Acked-by: NArnd Bergmann <arnd@arndb.de>
Acked-by: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/input/touchscreen/bu21013.txt

+* Rohm BU21013 Touch Screen
+
+Required properties:
+ - compatible              : "rohm,bu21013_tp"
+ - reg                     :  I2C device address
+
+Optional properties:
+ - touch-gpio              : GPIO pin registering a touch event
+ - <supply_name>-supply    : Phandle to a regulator supply
+ - rohm,touch-max-x        : Maximum outward permitted limit in the X axis
+ - rohm,touch-max-y        : Maximum outward permitted limit in the Y axis
+ - rohm,flip-x             : Flip touch coordinates on the X axis
+ - rohm,flip-y             : Flip touch coordinates on the Y axis
+
+Example:
+
+	i2c@80110000 {
+		bu21013_tp@0x5c {
+			compatible = "rohm,bu21013_tp";
+			reg = <0x5c>;
+			touch-gpio = <&gpio2 20 0x4>;
+			avdd-supply = <&ab8500_ldo_aux1_reg>;
+
+			rohm,touch-max-x = <384>;
+			rohm,touch-max-y = <704>;
+			rohm,flip-y;
+		};
+	};