ARM: dts: imx7s: Add DMA channels for CSPI peripherals

This adds the rx/tx DMA channels for CSPI peripherals. Channel numbers
are taken from i.MX7D RM Rev1 01/2018.
Peripheral types ID (7) is selected according to fsl,imx-sdma.yaml and
is similar to i.MX6 and i.MX8M platforms. Same goes for transfer
priority ID (last number).

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>