dt-bindings:iio:dac:ti,dac7512 yaml conversion

This one is a bit interesting because the binding was moved from
misc a while back, but the linux support for this device is
provided via the ad5446 DAC driver which doesn't currently
have a binding.

For now, lets just convert this file over, but we may want to
think about consolidating this with proper documentation of
the bindings for the other parts supported by the ad5446 driver.

As Daniel Mack does not seem to have been active since 2015,
I've put myself as maintainer of this binding for now.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201031134110.724233-15-jic23@kernel.org