ARM: dts: imx6sx: Fix SAI DMA index

According to sdma_peripheral_type in include/linux/platform_data/dma-imx.h
IMX_DMATYPE_SAI corresponds to index 24, so fix it accordingly.

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