iio: adc: stm32: make core adc clock optional by default

Analog clock input is mandatory on stm32f4. But newer version of
ADC hardware block allow to select either bus clock or asynchronous
clock, for analog circuitry.

So, make it optional by default, but enforce clk presence on stm32f4.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>