So far only USART drivers could use TX/RX DMA. This commit adds the
support for UART drivers as well (with the exception of serial console).
UART peripheral does not have timeout interrupt, so external polling
is required to flush the RX data from DMA buffers even if the buffer
is not yet full. The board layer can do this by calling
sam_serial_dma_poll function.
Signed-off-by: Michal Lenc <michallenc@seznam.cz>
7af7655234