iio: adc: ad7192: Add sysfs ABI documentation

Add initial ABI documentation for ad7192 adc sysfs interfaces.
Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio-adc-ad7192

+What:		/sys/bus/iio/devices/iio:deviceX/ac_excitation_en
+KernelVersion:
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Reading gives the state of AC excitation.
+		Writing '1' enables AC excitation.
+
+What:		/sys/bus/iio/devices/iio:deviceX/bridge_switch_en
+KernelVersion:
+Contact:	linux-iio@vger.kernel.org
+Description:
+		This bridge switch is used to disconnect it when there is a
+		need to minimize the system current consumption.
+		Reading gives the state of the bridge switch.
+		Writing '1' enables the bridge switch.