dt-bindings:iio:dac:maxim,max5821 yaml conversion

Simple txt to yaml conversion for this binding description.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Philippe Reynes <tremyfr@yahoo.fr>
Link: https://lore.kernel.org/r/20201031134110.724233-22-jic23@kernel.org