iio: st_pressure: Export sampling frequency for lps25h and lps331ap

Both devices are using the same iio_chan_spec to define which settings
are exported with sysfs. Both are properly configured to set/get
sampling frequency for pressure and temperature. They also properly
export available sampling frequencies. The only missing thing is
sampling_frequency sysfs file, which allows to set/get this property
from userspace.

Add sampling frequency to iio channel info mask, so sampling_frequency
file is properly exported using sysfs.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>