NFC: pn544: i2c: Add DTS Documentation

Describe the properties used by the PN544 NFC controller driver.
Signed-off-by: Clément Perrochaud <clement.perrochaud@nxp.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

Documentation/devicetree/bindings/net/nfc/pn544.txt

+* NXP Semiconductors PN544 NFC Controller
+
+Required properties:
+- compatible: Should be "nxp,pn544-i2c".
+- clock-frequency: IC work frequency.
+- reg: address on the bus
+- interrupt-parent: phandle for the interrupt gpio controller
+- interrupts: GPIO interrupt to which the chip is connected
+- enable-gpios: Output GPIO pin used for enabling/disabling the PN544
+- firmware-gpios: Output GPIO pin used to enter firmware download mode
+
+Optional SoC Specific Properties:
+- pinctrl-names: Contains only one value - "default".
+- pintctrl-0: Specifies the pin control groups used for this controller.
+
+Example (for ARM-based BeagleBone with PN544 on I2C2):
+
+&i2c2 {
+
+	status = "okay";
+
+	pn544: pn544@28 {
+
+		compatible = "nxp,pn544-i2c";
+
+		reg = <0x28>;
+		clock-frequency = <400000>;
+
+		interrupt-parent = <&gpio1>;
+		interrupts = <17 GPIO_ACTIVE_HIGH>;
+
+		enable-gpios = <&gpio3 21 GPIO_ACTIVE_HIGH>;
+		firmware-gpios = <&gpio3 19 GPIO_ACTIVE_HIGH>;
+	};
+};