ARM: dts: imx6qdl: Add label snvs_rtc

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

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