iio:st-sensors: Move exports into IIO_ST_SENSORS namespace

To avoid unnecessary pollution of the global symbol namespace move the
driver core and type specific core exports into their a new namespace
and import that where needed.

For more info see https://lwn.net/Articles/760045/



Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Denis Ciocca <denis.ciocca@st.com>
Link: https://lore.kernel.org/r/20220116180535.2367780-14-jic23@kernel.org