drivers: adc: stm32: support DMA when CONFIG_MEM_ATTR=n

Allow STM32 ADC driver to operate with DMA support even when
CONFIG_MEM_ATTR is disabled which happen when CONFIG_ARM_MPU is
intentionally disabled despite the CPU supports MPU.

By the way, remove some #ifdef directive on header files inclusion
that add noise in the header file inclusion section without any
benefit. Also remove inclusion of zephyr/arch/cache.h that is not
needed at all.

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>