ARM: dts: at91: Add label for sam9x5's internal RTC

This allows to simplify several machine device trees using this label
instead of duplicating the SoC's hierarchy.
Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Link: https://lore.kernel.org/r/20190812212757.23432-1-uwe@kleine-koenig.orgSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

arch/arm/boot/dts/at91sam9x5.dtsi

@@ -941,7 +941,7 @@ watchdog: watchdog@fffffe40 {
 				status = "disabled";
 			};
 
-			rtc@fffffeb0 {
+			rtc: rtc@fffffeb0 {
 				compatible = "atmel,at91sam9x5-rtc";
 				reg = <0xfffffeb0 0x40>;
 				interrupts = <1 IRQ_TYPE_LEVEL_HIGH 7>;