arm64: dts: ls1028a: Specify the DMA channels for the DSPI controllers

LS1028A has a functional connection to the eDMA module. Even if the
spi-fsl-dspi.c driver is not using DMA for LS1028A now, define the slots
in the DMAMUX for connecting the eDMA channels to the 3 DSPI
controllers.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>