ARM: dts: imx6sll: Make ssi node name same as other platforms

In imx6sll.dtsi, the ssi node name is different with other
platforms (imx6qdl, imx6sl, imx6sx), but the
sound/soc/fsl/fsl-asoc-card.c machine driver needs to check
ssi node name for audmux configuration, then different ssi
node name causes issue on imx6sll platform.

So we change ssi node name to make all platforms have same
name.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>