dt-bindings: iio: adc: tlc4541 - recover accidentally dropped binding doc

JC: Seems that I messed up applying the original driver patches, and
this file never actually made it into the tree.

I have picked up original Ack and Sign-off so as to record the
history.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200809111753.156236-13-jic23@kernel.org