ARM: dts: imx: correct sdma compatbile for imx6sl and imx6sx

The SDMA on imx6sl and imx6sx is more compatible with imx6q one than
imx35.  Let's use "fsl,imx6q-sdma" instead of "fsl,imx35-sdma", so that
SDMA ROM script on imx6sl and imx6sx can work for audio driver just like
the case of imx6q.

Reported-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Acked-by: Robin Gong <yibin.gong@freescale.com>