nfc: pn533: Add device tree documentation for i2c phy

Add pn533-i2c phy devicetree documentation
Signed-off-by: Michael Thalmeier <michael.thalmeier@hale.at>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

Documentation/devicetree/bindings/net/nfc/pn533-i2c.txt

+* NXP Semiconductors PN532 NFC Controller
+
+Required properties:
+- compatible: Should be "nxp,pn532-i2c" or "nxp,pn533-i2c".
+- clock-frequency: I²C work frequency.
+- reg: address on the bus
+- interrupt-parent: phandle for the interrupt gpio controller
+- interrupts: GPIO interrupt to which the chip is connected
+
+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 PN532 on I2C2):
+
+&i2c2 {
+
+	status = "okay";
+
+	pn532: pn532@24 {
+
+		compatible = "nxp,pn532-i2c";
+
+		reg = <0x24>;
+		clock-frequency = <400000>;
+
+		interrupt-parent = <&gpio1>;
+		interrupts = <17 IRQ_TYPE_EDGE_FALLING>;
+
+	};
+};