dt-bindings:iio:health:ti,afe4403: txt to yaml 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 requests tx_sup.
    I'll post a fix patch to change the driver to fix this as it makes
    little sense.

Andrew's email is bouncing so until someone else steps up I have
listed myself as maintainer for this binding.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201031184854.745828-8-jic23@kernel.org