drivers: adc: stm32U5 channel preselection

The stm32U5 LL function name differs from stm32H7 serie but must
still be enabled in the PCSEL.
This is done with the LL_ADC_SetChannelPreselection function
until the LL changes its name.

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