samv7/serial: allow TX/RX DMA for UART drivers

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 <[email protected]>

Documentation/platforms/arm/samv7/index.rst

@@ -284,10 +284,12 @@ The peripheral implements four timer counter modules, each supporting three inde
 Universal Synchronous Asynchronous Receiver Transceiver (USART)
 ---------------------------------------------------------------
 
-The MCU supports both UART and USART controllers. USART peripheral can be used with TX and RX DMA support.
-For RX DMA it is required to configure idle bus timeout value in ``CONFIG_SAMV7_SERIAL_DMA_TIMEOUT``.
-This option ensures data are read from the DMA buffer even if it is not full yet. DMA support is
-implemented only for USART peripheral and not for UART.
+The MCU supports both UART and USART controllers. These peripheral can be used with TX and RX DMA support.
+For RX DMA on USART, it is possible to configure idle bus timeout value in ``CONFIG_SAMV7_SERIAL_DMA_TIMEOUT``.
+This option ensures data are read from the DMA buffer even if it is not full yet. UART peripherals do not have
+this timeout support, therefore function :c:func:`sam_serial_dma_poll` should be called periodically to
+flush the DMA buffers. Boards can use common :c:func:`board_uart_rxdma_poll_init` function to initialize
+a timer triggering the poll.
 
 There are several modes in which USART peripheral can operate (ISO7816, IrDA, RS485, SPI, LIN and LON).
 Currently RS485 and SPI master are supported by NuttX.