ARM: dts: at91: sama5d2: Add DMA bindings for the flx3 SPI function

ARM: dts: at91: sama5d2: Add DMA bindings for the flx3 SPI function

Spare boards of duplicating the DMA bindings. Describe the flx3
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/20200514050301.147442-10-tudor.ambarus@microchip.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>