iio: accel: adxl372: Add support for FIFO peak mode

By default, if all three channels (x, y, z) are enabled, sample sets of
concurrent 3-axis data is stored in the FIFO. This patch adds the option
to configure the FIFO to store peak acceleration (x, y and z) of every
over-threshold event. When pushing to iio buffer we push only enabled
axis data.

Currently the driver configures adxl372 to work in loop mode.
The inactivity and activity timings  decide how fast the chip
will loop through the awake and waiting states and the
thresholds on x,y,z axis decide when activity or inactivity
will be detected.

This patch adds standard events sysfs entries for the inactivity
and activity timings: thresh_falling_period/thresh_rising_period
and for the in_accel_x_thresh_falling/rising_value.
Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Link: https://lore.kernel.org/r/20200810093257.65929-2-alexandru.tachici@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>