dt-bindings:iio:adc:dlg,da9150-gpadc yaml conversion

This is a small part of an MFD so perhaps ultimately it makes more
sense to document it with that MFD binding rather than separately.

In the meantime it's a straightforward conversion from txt to yaml.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Link: https://lore.kernel.org/r/20200909175946.395313-13-jic23@kernel.org