ARM: dts: imx6sl: 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: NFabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>

arch/arm/boot/dts/imx6sl.dtsi

@@ -569,7 +569,7 @@
 				#size-cells = <1>;
 				ranges = <0 0x020cc000 0x4000>;
 
-				snvs-rtc-lp@34 {
+				snvs_rtc: snvs-rtc-lp@34 {
 					compatible = "fsl,sec-v4.0-mon-rtc-lp";
 					reg = <0x34 0x58>;
 					interrupts = <0 19 IRQ_TYPE_LEVEL_HIGH>,