iio: temperature: mlx90632 Read sampling frequency

Allow users to read sensor sampling frequency to better plan the
application measurement requests.

Signed-off-by: Crt Mori <cmo@melexis.com>
Link: https://lore.kernel.org/r/0bd6d6d665b4bd39e4565f6f44cb1bdc03386e23.1663834141.git.cmo@melexis.com


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