dt-bindings:iio:potentiostat:ti,lmp91000: txt to yaml conversion.

There were a few parts of the example that did not conform to the
binding description and would not have worked with the Linux driver
as a result.  Fixed them whilst doing this conversion.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Cc: Matt Ranostay <matt.ranostay@konsulko.com>
Link: https://lore.kernel.org/r/20201031181242.742301-11-jic23@kernel.org