NFC: dts: st21nfca_i2c: Add DTS Documentation

Describe the properties used by the st21nfca NFC controller driver.

Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

diff --git a/Documentation/devicetree/bindings/net/nfc/st21nfca.txt b/Documentation/devicetree/bindings/net/nfc/st21nfca.txt
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+* STMicroelectronics SAS. ST21NFCA NFC Controller
+
+Required properties:
+- compatible: Should be "st,st21nfca-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
+- enable-gpios: Output GPIO pin used for enabling/disabling the ST21NFCA
+
+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 ST21NFCA on I2C2):
+
+&i2c2 {
+
+       status = "okay";
+
+       st21nfca: st21nfca@1 {
+
+               compatible = "st,st21nfca_i2c";
+
+               reg = <0x01>;
+               clock-frequency = <400000>;
+
+               interrupt-parent = <&gpio5>;
+               interrupts = <2 IRQ_TYPE_LEVEL_LOW>;
+
+               enable-gpios = <&gpio5 29 GPIO_ACTIVE_HIGH>;
+       };
+};