ARM: imx6: Align ssi nodes between mx6 variants

Since commit 98ea6ad2

 (ARM: dts: imx6: use imx51-ssi) the mx6 ssi is
compatible with imx51, so align all the mx6 variant ssi compatible strings as:

compatible = "fsl,<imx6-soc>-ssi", "fsl,imx51-ssi";

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>