dt-bindings: iio: adc: lltc,ltc2497 yaml conversion.

Very simple binding for this i2c device with a reference supply.

Added the #io-channel-cells property to allow for consumers.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Link: https://lore.kernel.org/r/20200809111753.156236-5-jic23@kernel.org