dt-bindings: iio: dac: Fix AD5686 references

dt-bindings: iio: dac: Fix AD5686 references

The example and filename use 'adi,ad5686', but the schema doesn't
document it. The AD5686 is also a SPI interface variant while all the
documented variants have an I2C interface. So let's update all the
references to AD5686 to AD5696.

Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Cc: Michael Auchter <michael.auchter@ni.com>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20210202215503.114113-1-robh@kernel.org