drivers: serial: uart_max32: Use cache to workaround DMA limitation

When using asynchronous API, transfer will fail if the source buffer is
located in a region that cannot be accessed by DMA. This could happen
when a buffer is declared const, and placed in flash memory, for
example.

Workaround this problem by loading the data into a set of temporary
caches before passing them to DMA.

Signed-off-by: Tahsin Mutlugun <Tahsin.Mutlugun@analog.com>