ARM: dts: at91: sama5d2: Add DMA bindings for the SPI and UART flx4 functions

Spare boards of duplicating the DMA bindings. Describe the flx4
DMA bindings in the SoC dtsi. Users that don't want to use DMA
for their flexcom functions have to overwrite the flexcom DMA
bindings in their board device tree.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20200514050301.147442-9-tudor.ambarus@microchip.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>