NFC: dts: st21nfcb_i2c: Add DTS Documentation

Describe the properties used by the st21nfcb NFC controller driver.
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

Documentation/devicetree/bindings/net/nfc/st21nfcb.txt

+* STMicroelectronics SAS. ST21NFCB NFC Controller
+
+Required properties:
+- compatible: Should be "st,st21nfcb_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
+- reset-gpios: Output GPIO pin used to reset the ST21NFCB
+
+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 BeagleBoard xM with ST21NFCB on I2C2):
+
+&i2c2 {
+
+	status = "okay";
+
+	st21nfcb: st21nfcb@8 {
+
+		compatible = "st,st21nfcb_i2c";
+
+		reg = <0x08>;
+		clock-frequency = <400000>;
+
+		interrupt-parent = <&gpio5>;
+		interrupts = <2 IRQ_TYPE_LEVEL_LOW>;
+
+		reset-gpios = <&gpio5 29 GPIO_ACTIVE_HIGH>;
+	};
+};