Add DMA configuration in stm32mp15x uart nodes by selecting dma direct
mode and alternate REQ/ACK dma protocol for uart.

DMA direct mode allows to bypass DMA FIFO. Each DMA request immediately
initiates a transfer from/to the memory. This allows USART to get data
transferred, even when the transfer ends before the DMA FIFO completion.

Default REQ/ACK DMA protocol consists in maintaining ACK signal up to the
removal of REQuest and the transfer completion.
In case of alternative REQ/ACK protocol, ACK de-assertion does not wait the
removal of the REQuest, but only the transfer completion.
Due to a possible DMA stream lock when transferring data to/from STM32
USART/UART, select this alternative protocol in STM32 USART/UART nodes.
Signed-off-by: NValentin Caron <valentin.caron@foss.st.com>
Signed-off-by: NErwan Le Ray <erwan.leray@foss.st.com>
Signed-off-by: NAlexandre Torgue <alexandre.torgue@foss.st.com>