iio: adc: stm32: allow to tune analog clock

Add new optional dt property to tune analog clock prescaler.
Driver looks for optional "st,max-clk-rate-hz", then computes
best approximation below that rate, using ADC internal prescaler.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>