NFC: trf7970a: Document the 'autosuspend-delay' DTS property

The trf7970a driver recently had support added for the 'autosuspend-delay'
property so document it.
Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

Documentation/devicetree/bindings/net/nfc/trf7970a.txt

@@ -12,6 +12,7 @@ Required properties:
 Optional SoC Specific Properties:
 - pinctrl-names: Contains only one value - "default".
 - pintctrl-0: Specifies the pin control groups used for this controller.
+- autosuspend-delay: Specify autosuspend delay in milliseconds.
 
 Example (for ARM-based BeagleBone with TRF7970A on SPI1):
 
@@ -29,6 +30,7 @@ Example (for ARM-based BeagleBone with TRF7970A on SPI1):
 		ti,enable-gpios = <&gpio2 2 GPIO_ACTIVE_LOW>,
 				  <&gpio2 5 GPIO_ACTIVE_LOW>;
 		vin-supply = <&ldo3_reg>;
+		autosuspend-delay = <30000>;
 		status = "okay";
 	};
 };