dt-bindings:iio:adc:nxp,lpc1850-adc yaml conversion

Most of the description in the original doc is effectively boilerplate
and does not bring much value so I have not carried it over into the yaml.

Added #io-channel-cells to simplify use of channels on this ADC by
consumer drivers.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Joachim Eastwood <manabian@gmail.com>
Link: https://lore.kernel.org/r/20200909175946.395313-10-jic23@kernel.org