drivers: serial: uart_stm32: support DMA when CONFIG_MEM_ATTR=n

Allow STM32 UART 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.

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