dt-bindings: iio: sx9324: Add input analog gain

Allow setting the configure the input analog gain.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220429220144.1476049-8-gwendal@chromium.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>