staging:iio:dac:max517 remove documentation

Whilst there is some useful info in here, it can be easily
obtained from datasheets. Some of the info should perhaps
be incorporated into a device tree bindings doc.
As this didn't move out of staging with the driver, I'm suggesting
we just drop it.  We don't generally carry per driver documentation
with the exception of non standard ABI which is not the case here.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Roland Stigge <stigge@antcom.de>
Link: https://lore.kernel.org/r/20200905174721.216452-2-jic23@kernel.org

drivers/staging/iio/Documentation/dac/max517

-Kernel driver max517
-====================
-
-Supported chips:
-  * Maxim MAX517, MAX518, MAX519
-    Prefix: 'max517'
-    Datasheet: Publicly available at the Maxim website
-               http://www.maxim-ic.com/
-
-Author:
-        Roland Stigge <stigge@antcom.de>
-
-Description
------------
-
-The Maxim MAX517/518/519 is an 8-bit DAC on the I2C bus. The following table
-shows the different feature sets of the variants MAX517, MAX518 and MAX519:
-
-Feature                              MAX517 MAX518 MAX519
---------------------------------------------------------------------------
-One output channel                   X
-Two output channels                         X      X
-Simultaneous output updates                 X      X
-Supply voltage as reference                 X
-Separate reference input             X
-Reference input for each DAC                       X
-
-Via the iio sysfs interface, there are three attributes available: out1_raw,
-out2_raw and out12_raw. With out1_raw and out2_raw, the current output values
-(0..255) of the DACs can be written to the device. out12_raw can be used to set
-both output channel values simultaneously.
-
-With MAX517, only out1_raw is available.
-
-Via out1_scale (and where appropriate, out2_scale), the current scaling factor
-in mV can be read.
-
-When the operating system goes to a power down state, the Power Down function
-of the chip is activated, reducing the supply current to 4uA.
-
-On power-up, the device is in 0V-output state.