dt-bindings:iio:afe:current-sense-amplifier: txt to yaml conversion.

Note this includes a fix in the example where we had *-mul instead of
*-mult.  The binding doc and driver agree that it should be *-mult

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Peter Rosin <peda@axentia.se>
Link: https://lore.kernel.org/r/20201031181242.742301-6-jic23@kernel.org