What:		/sys/bus/iio/devices/iio:deviceX/in_voltage_spi_clk_freq
KernelVersion:	4.14
Contact:	arnaud.pouliquen@st.com
Description:
		For audio purpose only.
		Used by audio driver to set/get the spi input frequency.
		This is mandatory if DFSDM is slave on SPI bus, to
		provide information on the SPI clock frequency during runtime
		Notice that the SPI frequency should be a multiple of sample
		frequency to ensure the precision.
		if DFSDM input is SPI master
			Reading  SPI clkout frequency,
			error on writing
		If DFSDM input is SPI Slave:
			Reading returns value previously set.
			Writing value before starting conversions.