iio:proximity:sx9310: Extract common Semtech sensor logic

Before adding new Semtech sensors, move common logic to all Semtech SAR
sensor in its own file:
- interface with IIO subsystem,
- interrupt management,
- channel access conrol,
- event processing.

The change adds a bidirectional interface between sx93xx and sx_common.

The change is quite mechanical, as the impacted functions are moved
and renamed.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20220101203817.290512-3-gwendal@chromium.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>