drivers: uart_stm32: add DCache support in async DMA mode

Adapt the driver to verify if DMA buffers are located in noncacheable
memory when DCache is activated, in order to avoid cache coherency issues.
This is required until manual cache coherency management is implemented.

Signed-off-by: Abderrahmane Jarmouni <abderrahmane.jarmouni-ext@st.com>