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>

arch/arm/boot/dts/imx6qdl.dtsi

@@ -658,7 +658,7 @@ snvs@020cc000 {
 				#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>,