dt-bindings:iio:adc:adi,ad7298 document bindings

The device has a tsens-busy pin, but it's both fiddly and currently
ignored by the Linux driver.  Given it's not clear whether the binding
should be an interrupt, or a GPIO I have left that out for now.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210401174112.320497-7-jic23@kernel.org