drivers: adc: stm32h7 adc driver has a range for oversampling ratio

On the stm32H7 family, there are ADC which have Oversampling ratio
of 10 bits OSVR[9:0] in the CFGR2 register. It means that oversampling
512x or 1024x are possible.
Other values are not allowed as the oversampling field of the struct
adc_sequence (adc.h) is 2^oversampling.

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