drivers/sensor/: lis2dux12: support FIFO modes

Support three different FIFO contents which are selectable through
a new DT property, fifo-mode-sel, which may be set to one of the
following values:

    - 0x0 # 1x Accelerometer @12bit and 1x temperature @12bit samples
    - 0x1 # 1x Accelerometer @16bit sample
    - 0x2 # 2x Accelerometer @8bit samples (previous and current)

Signed-off-by: Armando Visconti <armando.visconti@st.com>