iio: mma8452: add support for oversampling ratio

This adds the following sysfs files according to the iio ABI:

-rw-r--r--    4096 in_accel_oversampling_ratio
-r--r--r--    4096 in_accel_oversampling_ratio_available

Internally, the device knows about 4 different power modes that differ
in oversampling ratio (and power consumption). We just show the user
what oversampling ratio(s) is/are available, depending on the current
frequency.

The referenced table in the datasheets makes it easier to understand.
Signed-off-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Christoph Muellner <christoph.muellner@theobroma-systems.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>