iio: imu: st_lsm6dsx: add system power management support

Add system sleep power management support to st_lsm6dsx driver.
In particular during suspend phase each sensor is disabled and
hw fifo is configured in bypass in order to avoid subsequent
I/O operations. The patch has been tested on HiKey board device

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>