ARM: dts: imx6: use imx51-ssi

imx51-ssi and imx21-ssi are different IPs. imx51-ssi supports online
reconfiguration and needs this for correct interaction with SDMA. This
patch adds imx51-ssi before each imx21-ssi for all imx6 SoCs.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>