drivers: adc: stm32h7 adc driver using LL function

Change the access to the PCSEL register by using the LL function
because on the STM32H7xx soc, some devices have no PCEL register
especially on ADC3 of the stm32H723.
The LL function manages this difference.

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