iio: ABI: add DAC sysfs synchronous_mode parameter

Some DACs as ad3552r need a synchronous mode setting, adding
this parameter for ad3552r and for future use on other DACs,
if needed.

Signed-off-by: Angelo Dureghello <[email protected]>

Author: spectrum70

Documentation/ABI/testing/sysfs-bus-iio-dac

@@ -59,3 +59,10 @@ Description:
 		multiple predefined symbols. Each symbol corresponds to a different
 		output, denoted as out_voltageY_rawN, where N is the integer value
 		of the symbol. Writing an integer value N will select out_voltageY_rawN.
+
+What:		/sys/bus/iio/devices/iio:deviceX/out_voltage_synchronous_mode
+KernelVersion:	6.13
+Contact:	[email protected]
+Description:
+		This attribute allows a specific synchronization mode, mainly
+		intended for DACs where multiple synchronization methods are available.