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