drivers: i2c: stm32: support DMA when CONFIG_MEM_ATTR=n

Allow STM32 I2C driver v2 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 a #ifdef directive on header files inclusion
that adds noise in the header file inclusion section without any
benefit.

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