dt-bindings: iio: afe: add binding for current-sense-amplifier

Similar to current sense shunts, but an amplifier enables the use
of a smaller sense resistance.

Signed-off-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>