iio: sx9310: Support setting hysteresis values

Add support for setting the hysteresis as a shifted value of a channel's
proximity threshold. Each channel can have a different threshold, but
the hysteresis applies to all channels as a right shift factor.
Therefore, duplicate the hysteresis value across all channels and make
it depend on the channel's proximity threshold. This is sort of odd but
seems to work in practice as most of the time only one channel is used.

Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Cc: Daniel Campello <campello@chromium.org>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Gwendal Grignou <gwendal@chromium.org>
Cc: Evan Green <evgreen@chromium.org>
Link: https://lore.kernel.org/r/20201007011735.1346994-4-swboyd@chromium.org


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