iio: temperature: mlx90632 Add runtime powermanagement modes

The sensor can operate in lower power modes and even make measurements when
in those lower powered modes. The decision was taken that if measurement
is not requested within 2 seconds the sensor will remain in SLEEP_STEP
power mode, where measurements are triggered on request with setting the
start of measurement bit (SOB). In this mode the measurements are taking
a bit longer because we need to start it and complete it. Currently, in
continuous mode we read ready data and this mode is activated if sensor
measurement is requested within 2 seconds. The suspend timeout is
increased to 6 seconds (instead of 3 before), because that enables more
measurements in lower power mode (SLEEP_STEP), with the lowest refresh
rate (2 seconds).

Signed-off-by: Crt Mori <cmo@melexis.com>
Link: https://lore.kernel.org/r/be405068f081f2d518843897b13cd0289c280b5d.1663834141.git.cmo@melexis.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>