iio: add addac subdirectory

iio: add addac subdirectory

[ Upstream commit b62e2e1763cda3a6c494ed754317f19be1249297 ]

For IIO devices that expose both ADC and DAC functionality.

Signed-off-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Link: https://lore.kernel.org/r/20211205114045.173612-2-cosmin.tanislav@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Stable-dep-of: 4f9b80aefb9e ("iio: addac: stx104: Fix race condition when converting analog-to-digital")
Signed-off-by: Sasha Levin <sashal@kernel.org>