ARM: dts: imx6sx: Add label snvs_rtc

It may be useful to disable the internal snvs-rtc when an external rtc is
available. This patch adds a label so that dts files can disable it.

Based on a patch from Markus Pargmann for imx6qdl.dtsi.

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