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