ARM: dts: imx50: Pass the 'dma-cells' property

Pass the '#dma-cells' property in the sdma node, so that the following
build warning is gone:

  DTC     arch/arm/boot/dts/imx50-evk.dtb
arch/arm/boot/dts/imx50-evk.dtb: Warning (dmas_property): Missing property '#dma-cells' in node /soc/aips@60000000/sdma@63fb0000 or bad phandle (referred from /soc/aips@50000000/spba@50000000/ssi@50014000:dmas[0])

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>