dt-bindings:iio:health:ti,afe4404: txt to yaml conversion

This basically has same questions as for the afe4403.  We could combine
the two bindings, but as the drivers are separate and it would be a little
fiddly due to different buses let's keep the separating.
To repeat questions from the ti,afe4403 binding.

A few questions came up whilst converting this one.
1) What is actually required?
   - Checking Linux driver, interrupt is not, and the tx-supply could
     be supplied by a stub regulator as long as it's always on.
     As such I have reduced the required list to just compatible and reg.
2) What is the regulator called?
   - It's tx-supply in the binding doc, but the driver request tx_sup
     I will shortly send out a fix for the driver to match the binding
     doc which is the better choice of naming.

As Andrew's email is bouncing, I've put myself as temporary maintainer
for this binding until someone else steps up.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201031184854.745828-9-jic23@kernel.org