drivers: adc: driver for stm32wl configure sampling

This Commit makes the adc configuration correct
for using the adc with 12b conversion on the stm32wl.
TriggerSource must be set to SW.
The ADC clock must be disabled by clock gating during CPU1 sleep/stop.

Signed-off-by: Francois Ramu <francois.ramu@st.com>