iio: adc: stm32: make per instance bus clock optional

STM32F4 requires one clock per ADC instance for register access. But,
newer version of ADC hardware block have common bus clock for all
instances (per instance driver isn't responsible for getting it).
So, make it optional by default. Still, enforce it's required on STM32F4.

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