dts: bindings: nxp,edma: force dma-cells number to 2

To allow EDMA configuration with the help of the DT_INST_DMAS_*
and DT_DMAS_* macros, all that a consumer node needs to know
is which channel to configure and what MUX value needs to be
used. As such, this commit allows doing this by forcing the
dma-cells property to 2, each cell representing one of the
aforementioned properties.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>