dt-bindings:iio:dac:nxp,lpc1850-dac yaml conversion.

Very similar binding to that for the ADC on the same device.
Conversion from txt to yaml format.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Vladimir Zapolskiy <vz@mleia.com>
Link: https://lore.kernel.org/r/20201031134110.724233-23-jic23@kernel.org